Here is a brief description of the most common email protocols:
SMTP (Simple Mail Transfer Protocol) – an Internet standard for email transmission. It is a push mail protocol, which means that the sender initiates SMTP transfers. Most systems that send mail over the Internet use SMTP to send messages from one server to another. Two other protocols (POP3 and IMAP) are used for retrieving and storing email. SMTP by default uses TCP port 25.
POP (Post Office Protocol) – an Internet standard used by local email clients to retrieve emails from a remote server. It is a pull protocol, which means that the receiving system initiates the transfer. POP moves the message from the email server onto your local computer and deletes them from the server, although there is usually an option to leave the messages on the email server as well. POP by default uses TCP port 110.
IMAP (Internet Message Access Protocol) – an Internet standard used by local email clients to retrieve emails from a remote server. Like POP, IMAP is a pull protocol. IMAP generally leaves messages on the server until the user explicitly deletes them, which allows multiple clients to manage the same mailbox. IMAP by default uses TCP port 143.