From http://www.w3schools.com (Copyright Refsnes Data)
Email is one of the most important uses of TCP/IP.
When you write an email, you don't use TCP/IP.
When you write an email, you use an email program like Lotus Notes, Microsoft Outlook or Netscape Communicator.
Your email program uses different TCP/IP protocols:
The SMTP protocol is used for the transmission of e-mails. SMTP takes care of sending your email to another computer.
Normally your email is sent to an email server (SMTP server), and then to another server or servers, and finally to its destination.
SMTP can only transmit pure text. It cannot transmit binary data like pictures, sounds or movies.
SMTP uses the MIME protocol to send binary data across TCP/IP networks. The MIME protocol converts binary data to pure text.
The POP protocol is used by email programs (like Microsoft Outlook) to retrieve emails from an email server.
If your email program uses POP, all your emails are downloaded to your email program (also called email client), each time it connects to your email server.
The IMAP protocol is used by email programs (like Microsoft Outlook) just like the POP protocol.
The main difference between the IMAP protocol and the POP protocol is that the IMAP protocol will not automatically download all your emails each time your email program connects to your email server.
The IMAP protocol allows you to look through your email messages at the email server before you download them. With IMAP you can choose to download your messages or just delete them. This way IMAP is perfect if you need to connect to your email server from different locations, but only want to download your messages when you are back in your office.
From http://www.w3schools.com (Copyright Refsnes Data)