Internet Message Access Protocol (IMAP) and Post Office Protocol - Version 3 (POP3) are both designed for use with email clients such as Outlook, Eudora or Thunderbird. IMAP and POP3 let you download and view your incoming email messages. Outgoing messages always use the Simple Mail Transfer Protocol (SMTP). Neither IMAP or POP3 come into play when accessing email via Web interfaces such as Gmail or Yahoo! Mail.
POP3 connects to the email server using your user name and password and downloads all new incoming messages before disconnecting. Some email clients include the option to keep copies of your incoming messages on the server. The default behavior, however, is to delete messages from the server after they've been downloaded to your email program.
POP3 works well with slow connections since the structure and messages reside on your computer. If you prefer to keep messages on your computer for security purposes, archiving or due to lack of server space, POP3 is also preferable. Connecting with multiple computers, programs or devices to the same email account using POP3 is complicated. Specific settings may be required to ensure that each client application receives a copy of all messages. Computer failure can wipe out all your messages if no copy exists on the server and there's no backup.
IMAP is designed as an always-on email protocol. The email server stores messages, folders and sub-folders. Client applications make the initial connection with your user name and password and maintain an open connection for extended periods of time. Instead of downloading messages, your email client lets you work with messages directly on the server.
IMAP is best suited to DSL or broadband connections. Most IMAP clients do include an option to download messages so you can work offline. In offline mode, deleted, moved and copied messages are updated on the server when you reconnect. By default, IMAP works well with multiple devices, since each device or program is accessing the copies of your messages stored on the server. Hardware failure in your computer never threatens email messages with IMAP. Space, however, can be a consideration if you have a large number of messages and limited space allotted on the server.
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