When your mail arrives, unless you are one of the few whose computers have a permanent internet connection, the mail does not get delivered to your computer automatically. Mail gets delivered to an incoming mail server (also known als a POP3 server), which holds onto the mail until you dial in and you run your mail program. Your mail program picks up the mail. After you have downloaded your mail to your own computer, you can disconnect, freeing up your phone. Then you can read and respond to your mail while your are offline. When you are ready to send your messages, you can reconnect and transmit your mail. To send mail, your mail program has to take mail to an outgoing mail server (SMTP server = Simple Mail Transfer Protocol). It’s sort of like having a post office box rather than home delivery - you have to pick it up at the post office and also deliver your outgoing mail there.
If you use an online service like AOL, the mail server is the same computer you connect to when you dial in so that when you run your e-mail program, your mail is right there for you to read, your provider can drop outgoing messages directly in the virtual mail chute, and there is no seperate POP3 or SMTP server involved.
(Reference: J.R. Levine, C. Baroudi, M. Levine Young, The Internet for Dummies, 2000, IDG Books)