What is IMAP?

IMAP, or Internet Message Access Protocol, is a modern mail transport protocol that has replaced the older POP protocol almost entirely.

With an IMAP connection, changes to the inbox, sub folders and actual emails themselves are synchronized back to the mail server. This means that if a user deletes an email on one client, any other connected clients will mirror the changes. Similarly, sent messages will appear in the sent items folder on all other clients, and message flags (read, replied to, etc.) will also be synchronized. Emails are stored on the server, with only copies being downloaded to the email client. This means that email history is preserved, at all times.

IMAP works by flagging emails with different statuses, and these flags determine what the email server will do with the emails. For example, when a user who is using the IMAP protocol to access their mailbox deletes an email, the message is flagged as deleted upon the server, but it is not actually deleted. Instead, it is hidden by the email client, and all email clients currently connected to the IMAP server. It is entirely possible to restore delete emails for this reason.

IMAP also offers push capability, with new emails being delivered to the email client as soon as they are received by the server. However, in practice, depending upon the load on the mail server, there can be some lag in delivery. Microsoft ActiveSync performs much better in this regard, delivering new emails in a hosted exchange environment almost instantly.