The three most prevalent protocols related to e-mail are: POP, IMAP, and SMTP. These protocols can be classified into two distinct categories, incoming and outgoing.
Incoming protocols (deal with accepting mail)
Outgoing protocols (deal with sending mail)
POP - Stands for 'Post Office Protocol'. POP3 is the latest version of POP and has made version 1 and 2 nearly obsolete. POP4 has been outlined with a specification, but there has been little news on it's implementation. The advantage of POP is that it allows users to download e-mails with a client, and then to read and edit them while off-line. The process POP usually follows,
Whilst POP can leave retrieved messages on the mail server, it usually does not. While most email clients support both POP and IMAP, ISPs in the UK use POP more than IMAP. POP uses SMTP to send messages, APOP and other function for authentication, and TLS or SSL for encryption.
IMAP - Stands for 'Internet Message Access Protocol'. IMAP's latest version is version 4. Supported by all modern clients and webmail services, it's capable of editing and functioning online and off-line (just like with POP). There are fewer ISPs that support IMAP in comparison to POP.
The chief advantage that IMAP has over POP is on local networks, such as at a large business or a college campus. The advantage is the speed a user can access a message on a local network, IMAP is quicker than POP. IMAP is quicker because it stays connected to mail server permanently (while the retrieval client is open), whereas POP only connects for a small window of time and then disconnects.
Another advantage of IMAP is that it allows multiple clients to connect to an individual mailbox at the same time. Whereas, POP only allows one client to connect to an individual mailbox.
IMAP was previously named 'Interactive Mail Access Protocol' and 'Interim Mail Access Protocol'.
SMTP - SMTP can be used for sending and receiving e-mail. Generally, user mail clients such as Outlook Express only use SMTP for sending messages to a remote server, which is accessed later by a POP or IMAP enabled user mail client.