In order to handle email uniformly across networks like the Internet, the Simple Mail Transfer Protocol (SMTP) was designed and is the standard protocol used for email exchange. At the server level, SMTP is responsible for both sending and receiving email between relay hosts that route the mail through the network to its destination. At the client (user) level we associate SMTP with sending mail only, and the address of the outgoing mail server is often smtp.[thedomain].com. The incoming mail server address, however, might be named after a retrieval protocol, such as Post Office Protocol 3 (POP3), pop3.[thedomain].com, or Internet Message Access Protocol (IMAP), imap.[thedomain].com.

Internet Service Providers (ISPs) commonly provide its customers with a POP3 incoming mail server address. Authentication is required to collect mail from an incoming mail server and takes the form of the email address and associated password. This might be a proprietary password used just for mail, or the password assigned to the account. Only one client can access a POP3 mailbox at a time, and once mail is collected, it is automatically deleted off the server.

While POP3 is efficient, it isn’t always handy. The IMAP protocol allows mail retrieval from the incoming mail server while leaving copies on the server. This is convenient for people who collect mail from multiple clients or locations. For example, using IMAP one can collect personal email while at work to keep abreast of things, then can delete it off the work machine and collect it again from home to reply. The home email client can then be configured to delete the mail off the server.

Those who use personal digital assistants (PDAs), cell phones and other personal electronics with email access and IMAP support will also appreciate being able to check mail without having the process delete the mail from the incoming mail server. Though some mail can be quickly answered using a thumb keypad, longer replies are more conveniently typed from a standard computer at a later time. Using the IMAP protocol, mail on the incoming mail server can also be tagged or flagged to indicate various states or conditions. For example, collected mail can be tagged as having been read.

POP3 is most often associated with ISP mail service, while IMAP with Web-based email services such as FastMail™, the encrypted HushMail™ service, and ad-supported Gmail®. However, IMAP flexibility can be used with most POP3 accounts by utilizing an IMAP-supported email client. Most popular email clients today support both POP3 and IMAP, including Microsoft® clients and Mozilla® Thunderbird®.