POP3 is designed to support "offline" mail processing. POP3 is ideal for people who only access their email on a single computer such as at home or at work, but not both. When using POP3, mail is delivered to our server, and the user uses a mail "client" program (Outlook, Thunderbird, etc.) that connects to the server and downloads all of the pending mail to the user's own machine. Once delivered to your computer, the messages are deleted from the mail server (unless you configure your client to leave your mail on the server.)
IMAP is an especially convenient method of delivery for those who access their email on multiple computers. A growing number of email users have one machine at work and a different one at home, possibly also a laptop for travel. Like POP3, mail is delivered to our server, but the mail client (Outlook, Thunderbird, etc) does not copy it all at once and then delete it from the server. Messages on our server can be marked with various status flags (e.g. "deleted" or "answered") and can be moved to folders either on a local computer or on the server. They will stay on the server until explicitly removed by the user. This way any changes made to the messages can be viewed from any computer.
