E-mail mailbox types described

POP3, IMAP4, Exchange (aka MAPI), http, etc.

POP3 - The POP3 (Post Office Protocol Version 3) e-mail mailbox is the most popular and most simplistic mailbox type used. Most ISPs, like Comcast or Earthlink, give their customers POP3 mailboxes. POP3 mailboxes support end-users with intermittent connections, allowing these users to retrieve e-mail when connected and then to view and manipulate the retrieved messages without needing to stay connected. Although most e-mail software (like Outlook) has an option to leave mail on server, e-mail software using POP3 generally connect, retrieve all messages, store them on the user's PC as new messages, delete them from the server, and then disconnect. With POP3, once your e-mail software downloads the message, that message is removed from the server and therefore can't be viewed via webmail or on another computer. E-mail software usually "sends" mail using the SMTP protocol.

IMAP4 - The IMAP4 (Internet Message Access Protocol Version 4) e-mail mailbox is slightly more advanced than POP3. IMAP4 supports both connected and disconnected modes of operation. E-mail software (like Outlook) using IMAP4 generally leave messages on the server until the user explicitly deletes them. This and other facets of IMAP operation allow you access you mail from multiple locations: e-mail software, webmail, cellular phone, etc.