Mail.app doesn't support the IDLE IMAP command. This means that Mail.app needs to initiate a connection to check to see if there are new unread messages. The IDLE command allows the IMAP client to leave a imap connection open, and the server will notify the client when a new message arrives.
The IMAP IDLE plugin enables this functionality in Mail.app. This has worked pretty well for me. I have noticed one problem:
- The plugin attempts connections to accounts that have been Inactivated.
[via Hawk Wings]