• Reopening an application after it unexpectedly quits
• Mail may reset to default if it unexpectedly quits twice in row

This file is where all the account settings are stored. As a result, all the non-.Mac account settings are lost. If you have a .Mac account, that account would appear to have been preserved because Mail would set it up automatically using the System Preferences > .Mac/MobileMe settings.

You may want to make a backup copy of the ~/Library/Mail folder first (e.g. by dragging it to the Desktop while holding the Option (Alt) key down), just in case something else goes wrong while trying to solve the problem. This is where Mail stores the mail on your computer.

Now, there are at least three ways to restore the account settings:

1. Restore ~/Library/Preferences/com.apple.mail.plist from a backup if you have one. Mail shouldn’t be running while you do this.
2. Set up your mail accounts again (you may want to quit Mail and trash the new com.apple.mail.plist first to start over). If given the option to import existing mailboxes or something like that, don’t. Just enter the account information and Mail will automagically rediscover the data in ~/Library/Mail/ when done. You’ll also have to re-configure some of the Mail > Preferences settings. For spam-related security reasons, the first thing you should do is go to Preferences > Viewing and disable Display remote images in HTML messages if it’s enabled.
3. Mail may have renamed the old preferences file to com.apple.mail.plist.saved. If that’s the case, you may try trashing the newcom.apple.mail.plist and renaming the old com.apple.mail.plist.saved back to com.apple.mail.plist. Again, be sure Mail isn’t running while doing this. Given the circumstances, there exists the possibility that com.apple.mail.plist.saved became corrupt, but that often is not the case and the settings can usually be restored by just renaming the file back to com.apple.mail.plist.

As a side effect of re-creating com.apple.mail.plist, Mail might rename Outbox (which is where messages waiting to be sent are stored) toDelivered. The name of that mailbox is actually a misnomer, as it would contain messages (if any) that couldn’t be delivered for some reason. You can delete that mailbox if you wish.

Note: For those not familiarized with the ~/ notation, it refers to the user’s home folder. That is, ~/Library is the Library folder within the user’s home folder, i.e./Users/username/Library.