POP3 (Post Office Protocol 3) and IMAP (Internet Message Access Protocol) are two protocols used to access email.
The main difference between them are:
- POP3 works by downloading your mail to your computer
- IMAP works by keeping mail on the server.
Because POP3 downloads messages, it is most useful when email is read from only one computer. It also allows you to read your downloaded messages while offline (disconnected from the Internet). Note: Downloaded emails are usually deleted from the server by default, unless you select an option in your email client to 'Keep a copy on the server'.
IMAP is useful when you need to read your email from multiple computers, for example, at work and at home. IMAP allows you to access your messages directly from the mail server. It does not download messages in one go (like POP3 does) but on-demand, thus making access to the messages quicker.
For more information about the difference between the protocols, see Comparing two approaches to remote mailbox access: IMAP vs. POP.