Note: If the OS X user account has admin privileges when running Postbox, Postbox will automatically exclude IMAP mail folders while preserving IMAP mail filters via time Machine. So the instructions below no longer apply to admin accounts on Mac OS X.
Postbox's mail format is one-file-per-folder (also known as the "mbox" format) which can result in very large Time Machine backups.
To solve this problem, you can exclude the ImapMail folder from being backed up on Time Machine as this information is already backed up on the server.
The ImapMail folder can be found under your Postbox profile folder (see http://support.postbox-inc.com/forums/78362/entries/69230-postbox-profile-data for details).
To exclude files and folders from Time Machine:
- Open System Preferences, click on the Time Machine icon
- Click on the "Options..." button
- Click on the [+] button to add files and folders to be excluded