POP3 and IMAP are responsible for managing incoming e-mails and they work in different ways to access your e-mail messages. Therefore, they are considered mail access protocols.
On the other hand, the Simple Mail Transfer Protocol (SMTP) is responsible for transferring messages from server to server, or from e-mail client to server. Since this is the protocol that sends e-mail from an e-mail account, it is called the outgoing protocol.
In short, IMAP and POP3 allow you to receive e-mails and SMTP allows you to send messages.
IMAP versus POP3
As we have already mentioned, both protocols involve email retrieval. All modern servers support both protocols, although they work in different ways.
While the POP3 protocol assumes that your e-mail is accessed from only one application, IMAP allows simultaneous access by multiple clients. Therefore, IMAP is more suitable if you want to access your e-mail from different locations or if your messages are managed by multiple users.
On the other hand, POP3 downloads your emails to your local computer and then deletes them from the server. This reduces the space used by your e-mail account on your web server.
In summary, there are 3 e-mail protocols - SMTP, POP3 and IMAP. Each of them works on specific port numbers and operates in different ways. If you have problems connecting to an incoming or outgoing server, try using an alternative port number.
