The abbreviation POP3 in translation from English denotes the 3rd version of the mail protocol. The essence of this term lies in the fact that POP3 is a protocol designed to work with incoming mail. Using the POP3 protocol, the mail client, first of all, picks up the letters that came to the server, forwards them to your computer, and then you can work with the received correspondence in the mail client, and you do not have to additionally contact the mail server.
The POP3 protocol allows users to access their server and retrieve the accumulated letters and messages. It is important to know that the user can get the right to access the POP server from any point of access to the World Wide Web. For this user, you need to start a mail agent (UA) that works over the POP3 protocol, and then configure it to work with your mail server.
Thus, a separate personal computer is at the head of the POP model. This computer functions exclusively as a client of the guard system. Messages are delivered to the client using the POP protocol, while messages are still sent using SMTP. On the user's computer, there are two interface agents to the mail system: send (SMTP) and delivery (POP). This is what the POP3 protocol developer calls split UAs. The concept of separate agents is summarized in the POP3 specification.
The POP3 protocol contains information about three stages of the process of receiving correspondence:
- authorization
- transaction
- update
The authorization process begins after a connection is established between the server and the POP3 client. During this stage, the client performs its own identification with the server. In case of successful authorization, the server opens the user's mailbox and after that a transaction process takes place, during which the client sends a request to the server to issue certain information (for example, a list of mail messages) or asks the server to perform a specific action (for example, issue message). The communication session ends at the update stage.
In this lesson, we will look at another e-mail protocol by which mail clients "communicate" with mail servers.
I don’t think you will be installing a local mail client. But, it is on the example of a local mail client that it is easier to understand what the mail protocols and, in particular, the POP3 protocol are.
I want to make a reservation right away that we are considering protocols for receiving (retrieving) e-mail from mail servers IMAP(imap, i-map) and POP3(pop three). But we are not (yet) considering the protocol for transferring e-mail to mail servers. SMTP(es-em-ty-pee).
Although there is nothing special about the SMTP protocol, we will briefly touch on it a little later.
So, setting up e-mail reception using the POP3 protocol.
We will not invent anything new, but simply create one more account for our test address [email protected]
To do this, we launch our e-mail client program, left-click on the "name" of our only account " [email protected]", And then again left-clicking on the menu item for creating an e-mail account.
In the next window, fill in the fields in the same way as we filled in these fields when setting up an account via IMAP. Then we click on the "Continue" button.
In the next window, we are offered a choice over which protocol we will "pick up" our mail from the mail server. We click the left mouse button first on the "POP3" selection button, and then again with the left mouse button on the "Finish" button.
Thus, in the mail client, we have created another account with which we will connect to the mail server and collect mail using the POP3 protocol.
The "Inbox" folder is still empty, as the client checks the mail on the server once every 10 minutes. We will not wait 10 minutes and will check the mail on the mail server immediately. To do this, press the button of the "Get" menu item.
This is where we start looking at how POP3 mail works.
Mail from the mail server is completely downloaded to your personal computer. In other words, it is copied to your computer in full.
And, now, you can work with e-mail completely autonomously. The mail that is on your personal computer is now not connected in any way with the mail server, unlike the IMAP protocol.
But, before we proceed, let's change the name of the email account created to work with the mail server using the POP3 protocol.
The fact is that the program substitutes an email address for the account name. And since we have two accounts for one email address, we ended up with two identical names for different accounts.
In general, so as not to get confused, we change the name of the "account".
To do this, hover the mouse pointer over the name of our account " [email protected]"And click on it with the left mouse button. Similarly, click on the account settings menu item "View the settings for this account."
We will open a settings window for our accounts. We change the name of our "account". In order not to invent anything, I will simply add “( pop3)».
It is now easier to navigate our accounts.
Let's assume that we have already looked through all the email and we no longer need it. And we decided to delete all letters at once. To do this, left-click on any letter. Using the key combination "Ctrl + A" select all letters at once and press the "Del" key to delete.
All mail was deleted and moved from the Inbox to the Deleted Items folder.
And now ATTENTION !!! Two points.
Now, until a new letter arrives, the Inbox folder will be empty. It seems to go without saying. But!
Let's see what is really going on with our mail on the mail server, which we deleted from ourselves. Let's look through an account configured for IMAP and through.
All remote mail remained on the mail server. Although we don't get it anymore. Is this good or bad?
There are more minuses than pluses here. Although as anyone.
One of the "long" drawbacks is that the volume of letters will grow over time and can completely clog the mailbox. Letters just stop coming.
One of the "big" advantages is that even if we completely delete our mail on the local computer, we will always know that we have a kind of "backup" of all our emails
Let's now change one parameter on the server.
To do this, we will enter the account settings view.
Left-click on the account name " [email protected](pop3) ", then on the item" View account settings ", then on the item" Server settings ".
Uncheck the box next to "Leave a copy of messages on the server." Click the "OK" button.
Now let's see what's with our mail on the mail server in our mailbox.
As we can see, all mail was deleted immediately after we unchecked the "Leave a copy of messages on the server" option.
Is this good or bad?
There is a very big plus here. You don't need to worry about your mailbox on the server. It just never overflows, and letters will always come.
One of the big drawbacks is that if your computer fails, then all mail will simply be lost, unless, of course, backups were made.
But, on the other hand, if there is no backup copy of something, then this “something” cannot be restored. And it doesn't matter what it is - mail, photographs, documents, etc.
Basically, in most cases, the account in the mail client for receiving mail from the mail server is configured using the POP3 protocol, without leaving mail in the mailbox on the server.
But of course, no one ever creates two accounts for the same email address. Although, as you can see, such a possibility exists. And I made two entries for illustrative purposes only.
That's basically all about the IMAP and POP3 protocols.
In the next lesson, we will learn how to "bind" email by working with a mailbox through the Web interface. We will also consider several different ways to insert pictures. 
In conclusion, I want to congratulate all my readers on the coming New Year and wish everyone health, success and happiness in their personal lives.
That's all for today. Good luck and creative success to everyone. 🙂
Post Office Protocol (POP) is a protocol for delivering mail to a user from a mailbox on a POP mail server. Many POP concepts, principles, and concepts look and function similar to SMTP. POP commands are almost identical to SMTP commands, differing in some details. The figure depicts the POP client-server model. The POP server sits between the user agent and mailboxes.
Currently, there are two versions of the POP protocol - POP2 and POP3, which have approximately the same capabilities, but incompatible with each other. The fact is that POP2 and POP3 have different protocol port numbers. There is no link between them, similar to the link between SMTP and ESMTP. The POP3 protocol is not an extension or modification of POP2 - it is a completely different protocol. POP2 is defined in RFC 937 (Post Office Protocol-Version 2, Butler, et al, 1985), and POP3 is defined in RFC 1225 (Post Office Protocol-Version 3, Rose, 1991). Next, we will briefly consider POP in general and in more detail - POP3. PORZ is developed taking into account the specifics of mail delivery to personal computers and has appropriate operations for this.
Purpose of the POP3 protocol
Previously, mail messages on most networks were delivered directly from one computer to another. And if the user often changed work computers or one computer belonged to several users, there were certain problems. Nowadays, it is generally accepted to deliver a message not to the user's computers, but to special mailboxes of the organization's mail server, which works around the clock (turned on).
Description of the POP3 protocol
The design of the POP3 protocol allows the user to access his mail server and retrieve the accumulated mail for him. The user can access the POP server from any Internet access point. At the same time, he must launch a special mail agent (UA) that works under the POP3 protocol, and configure it to work with his mail server. So, at the head of the POP model is a separate personal computer that works exclusively as a client of the mail system (server). We also emphasize that messages are delivered to the client using the POP protocol, but are still sent using SMTP. That is, there are two separate interface agents to the mail system on the user's computer - delivery (POP) and sending (SMTP). The developers of the POP3 protocol call this situation "split UAs". The concept of separate agents is briefly discussed in the POP3 specification.
The POP3 protocol specifies three stages in the mail receipt process: authorization, transaction, and renewal. After the POP3 server and client have established a connection, the authorization stage begins. At the authorization stage, the client identifies itself to the server. If the authorization is successful, the server opens the client's mailbox and the transaction phase begins. In it, the client either requests information from the server (for example, a list of mail messages), or asks him to perform a specific action (for example, issue a mail message). Finally, at the update stage, the communication session ends. Table 7 lists the POP3 protocol commands required for an Internet-based minimal configuration implementation.
Table 5. POP Version 3 Commands (For Minimal Configuration)
Team
Description
USER Identifies the user with the specified name
PASS
Specifies the password for the client-server pair
QUIT
Closes the TCP connection
STAT
The server returns the number of messages in the mailbox plus the size of the mailbox
LIST
The server returns message IDs along with message sizes (the command parameter can be a message ID)
RETR
Retrieves a message from a mailbox (requires the message id argument)
DELE
Marks a message for deletion (requires the argument - message id)
NOOP
The server returns a positive response, but takes no action
LAST
The server returns the largest message number that has been previously accessed
RSET
Cancels the deletion of a message previously marked with a DELE command
Several commands are defined in the POP3 protocol, but only two responses are given to them: + OK (positive, similar to ACK confirmation message) and -ERR (negative, similar to NAK not acknowledged message). Both responses confirm that the server has been contacted and that it is responding to commands at all. Typically, each answer is followed by a meaningful verbal description. RFC 1225 provides examples of several typical POP3 sessions. Now we will look at several of them, which will make it possible to catch the sequence of commands in the exchange between the server and the client.
After the program has established a TCP connection with the POP3 protocol port (official number 110), it is necessary to send the USER command with the username as a parameter. If the server's response is + OK, you need to send the PASS command with the password of this user:
CLIENT: USER kcope
ERVER: + OK
CLIENT: PASS secret
SERVER: + OK kcope "s maildrop has 2 messages (320 octets)
(There are 2 messages in kcope mailbox (320 bytes) ...)
POP3 transactions
The STAT command returns the number of messages and the number of bytes in messages:
CLIENT: STAT
SERVER: + OK 2 320
The LIST command (without a parameter) returns a list of messages in the mailbox and their sizes:
CLIENT: LIST
SERVER: + OK 2 messages (320 octets)
SERVER: 1 120
SERVER: 2 200
SERVER:. ...
The LIST command with a parameter returns information about the specified message:
CLIENT: LIST 2
SERVER: + OK 2 200 ...
CLIENT: LIST 3
SERVER: -ERR no such message, only 2 messages in maildrop
The TOP command returns the title, an empty line, and the first ten lines of the message body:
CLIENT: TOP 10
SERVER: + OK
SERVER:
(the POP server sends the message headers, a blank line and the first ten lines of the message body)
SERVER:. ...
CLIENT: TOP 100
SERVER: -ERR no such message
The NOOP command does not return any useful information other than a positive response from the server. However, a positive response means that the server is in communication with the client and is waiting for requests:
CLIENT: NOOP
SERVER: + OK
The following examples show how the POP3 server performs actions. For example, the RETR command retrieves the message with the specified number and places it in the local UA buffer:
CLIENT: RETR 1
SERVER: + OK 120 octets
SERVER:
(The POP3 server sends the entire message)
SERVER:. ... ... ... ... ...
The DELE command marks the message to be deleted:
CLIENT: DELE 1
SERVER: + OK message 1 deleted ...
(message 1 deleted)
CLIENT: DELE 2
SERVER: -ERR message 2 already deleted
message 2 has already been deleted)
The RSET command removes the deletion marks from all previously marked messages:
CLIENT: RSET
SERVER: + OK maildrop has 2 messages (320 octets)
(in a mailbox 2 messages (320 bytes))
As you might expect, the QUIT command closes the connection to the server:
CLIENT: QUIT
SERVER: + OK dewey POP3 server signing off
CLIENT: QUIT
SERVER: + OK dewey POP3 server signing off (maildrop empty)
CLIENT: QUIT
SERVER: + OK dewey POP3 server signing off (2 messages left)
Note that messages marked for deletion are not actually deleted until after the QUIT command is issued and the upgrade phase begins. At any time during the session, the client has the opportunity to issue an RSET command, and all messages marked for deletion will be restored.
In this article, we will talk about the difference between two mail protocols - POP and IMAP - and how they can be linked to each other.
Motivational picture J
Postal protocolPOP3
used by only to download new messages from the mail server. It allows the mail client to connect to the server only for the amount of time it takes to download messages.
Simultaneous connection of several mail clients to a specific mailbox is not allowed (only one connection is allowed).
When using postal protocolIMAPthe connection is not terminated as long as you are working with the mailbox in the mail program; downloading messages from the server is performed only at the request of the mail client.
Allowed simultaneous access of several mail clients to the mailbox, and each of them can track the changes made by other connected clients, as well as the status of all messages (read, reply sent, deleted).
Both of these protocols perform identical tasks for working with mail, only Pop 3 allows you to download all files at the same time, and IMAP - list the files first and then– the files themselves as needed.
One of the common problems when using protocols Pop 3 and IMAP , lies in the fact that many developers are trying to implement these protocols under the same interface and with the same logic. This is fundamentally wrong. The fact is that when using the protocol IMAP we can ship not all letters, but only unread ones for example. In order to implement data transfer, we used the library in the project MailBee. NET ... An example of the implementation of the technique using this library, you can look at... The example at the link above shows the implementation of the protocol POP 3 and IMAP using the factory method pattern ( Factory Method ). In one of these implementations, we used for the uniqueness of the letter UID (unique identifier of the letter on the server). The point is that for IMAP and POP 3 these identifiers are different.
RFC 1939(POP3): The unique-id of a message is an arbitrary server-determined string, consisting of one to 70 characters in the range 0x21 to 0x7E, which uniquely identifies a message within a maildrop and which persists across sessions.
RFC 3501(IMAP): (Unique Identifier (UID) Message Attribute is) a 32-bit value assigned to each message, which when used with the unique identifier validity value (see below) forms a 64-bit value that MUST NOT refer to any other message in the mailbox or any subsequent mailbox with the same name forever.
Let's see examples of the difference UIDs ... Examples of differences for POP 3:
pop. meta. ua − UID8863-1253275464
pop3.ukr.net− 1356790973352505619
Example UIDs for IMAP:
imap.ukr.net− 1432
It is theoretically possible to write a mail server that supports both IMAP and POP 3, and use the same UID for both protocols, but I don't know of any server that will actually do this. In practice, you must treatPOP 3-identifiers and IMAP-identifiers as unbound values... It seems that everything is logical, but what if we worked with one box that supports and POP 3 and IMAP ... And in your program you give the opportunity to indicate to the user which protocol he wants to use to receive mail. If we used, for example, pop 3.ukr. net and then decided to switch to using imap. ukr. net then we will get a bunch of problems on our heads. Because if you look at the example above for the difference in UID for these two protocols for ukr. net then we will see the main problem. Namely: duplication of some letters. If the user does not care about the fact of downloading letters to his disk, then this fact can be neglected, but if the fact of mail delivery is significant, this fact cannot be ignored. Therefore, many developers implement work with the protocol IMAP as with the protocol POP 3.
How can you "make friends" these two protocols? After much deliberation with colleagues at work and an unsuccessful Internet search for solutions to link letters that are received via the protocol POP 3, with letters that are received through the protocol IMAP , we came to a solution how you can defeat the binding of letters if they have attachments. First of all, we need to create a database for storing the identifier of the mail settings (for example, a link to the table of mail settings, which stores the address, port, password, etc.), a unique message identifier UID and the date the letter was received. This method will help us identify messages received from a specific mail server for a specific user.
The next step is to create a table for storing hash files that go in the letter ( attachment ). In order to calculate the hash, we can use the standard methods available in. NET Framework ... For example, to use MD 5, we will use the class MD 5 CryptoServiceProvider () or another hash algorithm availableHashAlgorithm on MSDN , or, if you wish, you can write your own algorithm. The main point is that if you store attachments that go in letters, you can only store those that are not duplicated. How it works: you downloaded a letter from the server, calculated its hash, checked if there is one in the hash table, if there is no such file in this table, we save the calculated hash for this file in the database, so that when switching the protocol, we do not load this file again ... After that, if we use the protocol IMAP , we can notify the server that we have read this letter, and at the next request it will not return it to us.
A huge disadvantage of this approach− the need to download the received letters in full, this may take a long time, depending on the bandwidth of the Internet channel. In this case, we align the protocol POP 3 with IMAP and we can use only a small part of the possibilities that the modern protocol provides us IMAP ... But we are dealing with the problem of storing duplicates. If you have the option not to consider switching from POP 3 to IMAP within one mailbox, this problem will not affect you. You will be able to use the full power of the protocol IMAP , and for some cases leave POP 3 as it is.
Note: some developers might be thinking why, for such purposes, do not sort it out by date of receipt. DataReceived. In the MailBee library. Net in class MailMessage that stores information about the letter, there is a field DateReceived , which contains the date of receipt of the letter by mailMailMessage.DateReceived Property ... But there is a problem with this field: it is not suitable for the protocol IMAP. For IMAP it is necessary to use the propertyEnvelope.DateReceived Property , but if the information is not available for some reason, then both of these properties with DateReceived will containMinValue... Therefore, even if we have DateReceived is not equal to the minimum date, it is not a fact that after changing the protocol from POP 3 to IMAP this date will be identical. One of the problems that was previously with MailBee. NET is that the date is DateReceived differed by one hour from real timeIncorrect Date Received Time ... Therefore, once again I would like to advise you that if it is possible to consider these two protocols as separate unique protocols, then you will have one less problem. Otherwise, you will have to link these two protocols, which is fraught with "crutches", which, if they work, then with a huge stretch.
This article covers the most commonly used email protocols on the Internet - POP3, IMAP, and SMTP. Each of them has a specific function and way of working. The content of the article explains which configuration is best for the specific needs of the user when using an e-mail client. It also reveals the answer to the question of which protocol supports e-mail.
What is POP3?
Version 3 (POP3) is a standard postal protocol used to receive e-mail from a remote server to a local mail client. Allows you to download messages to your local computer and read them even when the user is offline. Please note that when you use POP3 to connect to your account, messages are downloaded locally and deleted from the email server.
By default, the POP3 protocol runs on two ports:
port 110 is the unencrypted POP3 port;
port 995 - this must be used if you want to securely connect to POP3.
What is IMAP?
Internet Messaging Access Protocol (IMAP) is an e-mail retrieval protocol that is used to access e-mail on a remote web server from a local client. IMAP and POP3 are the two most commonly used protocols for receiving emails and are supported by all modern email clients and web servers.
POP3 assumes that your email address is only accessible from one application, while IMAP allows you to log in from multiple clients at the same time. This is why IMAP is better if you are going to access your email from different places or if your messages are managed by multiple users.
IMAP runs on two ports:
Port 143 is the default unencrypted IMAP port.
port 993 - this must be used if you want to connect securely using IMAP.
What is SMTP?
Protocol is a standard protocol for sending e-mail over the Internet.
SMTP works on three ports:
port 25 is unencrypted by default;
port 2525 - it opens on all SiteGround servers if port 25 is filtered (for example by your ISP) and you want to send unencrypted emails using SMTP.
port 465 - This is used if you want to send messages securely using SMTP.
What are the protocols for e-mail exchange? Concepts and terms
The term "email server" refers to the two servers required to send and receive emails, that is, SMTP and POP.
The Incoming Mail Server is the server associated with your email account. There can be no more than one incoming mail server for it. To access incoming messages, you need an email client - a program that can receive email from an account, allowing the user to read, forward, delete, and reply to messages. Depending on your server, you can use a dedicated email client (such as Outlook Express) or a web browser. For example, Internet Explorer is used to access email-based accounts. Emails are stored on the incoming mail server until they are downloaded. Once you have downloaded your mail from the mail server, you will not be able to do it again. To download the data successfully, you need to enter the correct settings in your email program. Most of the incoming mail servers use one of the following protocols: IMAP, POP3, HTTP.
Outgoing mail server (SMTP)
This is a server used only for sending emails (for transferring them from your email client program to the receiver). Most outgoing mail servers use Protocol) to send mail. Depending on your network settings, the outgoing mail server may belong to your ISP or the server on which you set up your account. Alternatively, you can use a subscription based SMTP server that will allow you to send emails from any account. Due to spam issues, most outbound mail servers will not allow you to send emails unless you are logged into your network. An open relay server will allow you to use it to send emails, whether you belong to its network group or not.
Email ports
For networks, a port refers to the endpoint of a logical connection. The port number determines its type. The following are the default email ports:
POP3 - port 110;
IMAP - port 143;
SMTP - port 25;
HTTP - port 80;
secure SMTP (SSMTP) - port 465;
secure IMAP (IMAP4-SSL) - port 585;
IMAP4 over SSL (IMAPS) - port 993;
Secure POP3 (SSL-POP) - port 995.
Email protocols: IMAP, POP3, SMTP and HTTP
Basically, the protocol refers to the standard method used at each end of the communication channel. To deal with email, you must use a dedicated client to access the mail server. In turn, they can exchange information with each other using completely different protocols.
IMAP protocol
IMAP (Internet Messaging Access Protocol) is a standard protocol for accessing email from your local server. IMAP is a client / server protocol in which email is received and the data is stored by your internet server. Since it only requires a little data transfer, it works well even on slow connections, such as when connecting with a modem. When trying to read a specific email message, the client downloads data from the server. You can also create and manage folders or mailboxes on the server, delete messages.
POP3 protocol
Post Office Protocol 3 (POP) e-mail protocol provides a simple, standardized way for users to access mailboxes and download messages to their computers.
With POP, all your e-mails will be downloaded from the mail server to your local computer. You can also leave copies of your emails on the server. The advantage is that after downloading your messages, you can disconnect your internet connection and read your e-mail at your leisure without incurring additional communication costs. On the other hand, with this protocol, you receive and download a lot of unwanted messages (including spam or viruses).
SMTP protocol
The protocol is used by the Mail Transfer Agent (MTA) to deliver e-mail messages to a specific recipient server. SMTP can only be used to send emails, not to receive them. Depending on the settings of your network or Internet service provider, you may only use the SMTP protocol under certain conditions.
HTTP protocols
HTTP is not a protocol designed for email communication, but it can be used to access your mailbox. It is also often referred to as web email. It can be used to compose or receive emails from your account. Hotmail is a good example of using HTTP as the email protocol.
Managed file transfers and networking solutions
Your ability to send and receive email is mainly due to three TCP protocols. These are SMTP, IMAP and POP3.
SMTP
Let's start with SMTP because its main function is different from the other two. SMTP, or Simple Mail Transfer Protocol, is mainly used to send email from an email client (such as Microsoft Outlook, Thunderbird, or Apple Mail) to an email server. It is also used to relay or forward mail messages from one mail server to another. This is necessary if the sender and the recipient have different email providers.
SMTP, which is specified in RFC 5321, uses port 25 by default. It can also use port 587 and port 465. The latter, which was introduced as the port of choice for secure SMTP (a.k.a. SMTPS), is deprecated. But in reality, it is still used by several postal service providers.
POP3
The Post Office Protocol, or POP, is used to retrieve e-mail messages from the latest version that is widely used is version 3, hence the term "POP3".
POP version 3 specified in RFC 1939 supports extensions and several authentication mechanisms. Authentication features are required to prevent attackers from gaining access to user messages.
The POP3 client receives email as follows:
connects to the mail server on port 143 (or 993 for SSL / TLS connections);
retrieves email messages;
serves to connect before closing the mail client application and download messages on demand.
connects to the mail server on port 110 (or 995 for SSL / TLS connections);
deletes copies of messages stored on the server;
disconnects from the server.
Although POP clients can be configured so that the server can continue to store copies of downloaded messages, the steps above are common practice.
IMAP
IMAP, especially the current version (IMAP4), is a more complex protocol. This allows users to group related messages and place them in folders, which in turn can be organized hierarchically. It also features message flags that indicate whether a message has been read, deleted, or received. It even allows users to search server mailboxes.
Operation logic (imap4 settings):
Please note that messages are not deleted on the server. This can have serious consequences. The IMAP specifications can be found in RFC 3501.
Choosing between IMAP and POP3
Since the basic function of SMTP is fundamentally different, the dilemma of choosing the best protocol usually involves only IMAP and POP3.
If storage space on the server is important to you, then choose POP3. A server with limited memory is one of the main factors that can make you support POP3. Because IMAP leaves messages on the server, it can consume memory space faster than POP3.
If you want to access your mail at any time, then IMAP is the best choice. There is one good reason why IMAP was designed to store messages on a server. It is used to search for messages from multiple devices - sometimes even simultaneously. So if you have an iPhone, Android tablet, laptop, and desktop and want to read email from any or all of these devices, then IMAP is your best bet.
Synchronization is another benefit of IMAP. If you're accessing your email messages from multiple devices, chances are you'll want them all to reflect whatever actions you've taken.
For example, if you read messages A, B, and C, you want them to be marked as “read” on other devices as well. If you deleted letters B and C, then you want the same messages to be deleted from your mailbox on all gadgets. All of these syncs can only be achieved if you are using IMAP.
Because IMAP allows users to organize messages in a hierarchical order and place them in folders, it helps users to better organize their correspondence.
Of course, all IMAP functionality comes at a price. These solutions are more difficult to implement, and ultimately the protocol consumes a lot more CPU and RAM, especially when it is performing the synchronization process. In fact, high CPU and memory usage can happen on both the client side and the server side if there are a ton of messages to sync. From this point of view, the POP3 protocol is less expensive, although less functional.
Privacy is also one of the concerns that will be highly dependent on end users. They would generally prefer to download all emails and not leave copies of them on an unknown server.
Speed is an advantage that varies and depends on the situation. POP3 has the ability to download all email messages when connected. And IMAP can, if necessary (for example, with insufficient traffic), download only message headers or certain parts and leave attachments on the server. Only when the user decides that the remaining parts are worth downloading will they become available to him. Therefore, IMAP can be considered faster.
However, if all messages on the server have to be downloaded every time, then POP3 will work much faster.
As you can see, each of the described protocols has advantages and disadvantages. It's up to you to decide which features or capabilities are more important.
Also, the preferred way to access the e-mail client determines the preference of the protocol. Users with only one machine and using webmail to access their new emails will appreciate POP3.
However, users who exchange mailboxes or access their emails from different computers will prefer IMAP.
Spam firewalls with SMTP, IMAP and POP3
Most spam firewalls only deal with and protect SMTP. The servers send and receive SMTP email and they will be checked by the spam firewall at the gateway. However, some spam firewalls provide the ability to protect POP3 and IMAP4 when external users need these services to access their email.
SMTP firewalls are transparent to end users; there are no configuration changes for clients. Users still receive and send email messages to the email server. For example, Exchange or Dominos must configure proxy-based message routing to the firewall when sending e-mail, as well as provide the ability to send mail from the firewall.
