Erase deleted messages from the Trash folder in Apple Mail
You may purge or erase messages in your Trash folder manually, automatically, or both.
Until messages have been purged or erased from the Trash folder, they are still available to read and/or recover. Once they have been purged, they have been permanently removed from Apple Mail and the mail server where they may have been originally stored.
Manually
To erase messages from the Trash folder manually, choose Mailbox > Erase Deleted Messages > In All Accounts.
Automatically
Apple Mail will also automatically erase all mail messages in the Trash folder. This is set up in Mail's Preferences for each individual account. In the Preferences, choose an account and then the Mailbox Behaviors tab. Select one of the options for the "Permanently erase deleted messages" preference - any choice other than "Never" will result in Mail automatically erasing messages in the Trash on a schedule.
However, as of Mac OS X Tiger, we have found that messages are not deleted automatically if you leave Apple Mail running all of the time. You may be more likely to do this if you leave your computer on at all hours (only letting it go into sleep mode), or you perpetually carry a laptop with you (also in sleep mode) without shutting it down.
We haven't carefully studied exactly what causes Mail to run its automatic cleanup routine for deleted items, but if you quit it and re-launch the program, that should purge messages that match your preference settings (one day, one week, or one month old, or any messages found in the Trash when Mail is quit).
