IMAP, or Internet Message Access Protocol, is a specification for Internet mail access. IMAP differs from other mail systems in that messages are manipulated directly on the email server, rather than downloaded to the user's local machine first. This allows the user to access mail from a variety of locations and machines. This process is what differentiates IMAP from the other primary email protocol in use, POP3. In POP3, email is delivered to the user's computer and manipulated locally. Depending on server settings, the email is usually deleted from the server computer after being transferred to the user.
Advantage: Synchronization
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IMAP can be accessed on the Web without an email client. Because all changes take place at a central location, the status of emails and folders is maintained between sessions, even if the user changes computers. Folder actions, read or unread status, and other manipulations appear the same on any computer, in any location. This means email can be checked from any computer at any time, and folders and message status are synchronized. Because emails only exist on the server in the IMAP protocol, they are safe if the computer is lost or destroyed, unlike with POP3.
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Advantage: Easy Migration
Migrating from one computer to another does not required downloading all messages again, since they are manipulated remotely. Little to no setup is required in this situation, since settings such as spam filters are saved. If hard drive space is at a premium, IMAP is ideal, since the data is stored remotely, and backup is handled remotely as well.
Disadvantages: Performance and Maintenance
IMAP can be complex to maintain, and thus some hosts do not support the protocol. It also uses the host's hard drive space, and most enforce storage quotas for their users. IMAP can be slower to load messages than IMAP. POP3 emails are manipulated rapidly, since they exist on the user's local machine. IMAP mail is only available when connected and is not available when offline. In addition, some email programs have difficulty supporting IMAP, though browser-based solutions are often available.