Corrupt Mac Mail

You may just need to repair permissions via Disk Utility, but it sounds like the title of your post has nailed it. Tho, more specifically, it's probably Mail's preference file. In your Users/yourname/Library/Preferences folder is a file called com.apple.Mail.plist. It most likely needs to be thrown out.

Quit Mail before you throw this file away, and note that you may have to set up your email accounts again next time you launch Mail. Although once you do that, your messages should show up the way they were before (your actual email lives in a folder called Mail in your user account's Library folder, separate from the preference file)

I suggest you be safe and back up your Users/yourname/Library/Mail folder before you do anything, of course. You can always reimport it later using Mail's Import function from the File menu.
 
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