Mail keeps crashing because: "Mailbox cannot access your mailbox information because it’s already in use."

Thanks for the tip. I set up a second user on my Mac and set up all my mail (3 separate accounts) in the new user and it's operating just fine. So it's just an issue with my admin account. Where do I go from here?

I've shut down and restarted several times. I even erased all my mailbox data, removing all my accounts, and even with an empty mailbox I'm getting the same message.

Here's the exact message:

Spoke with Apple on the phone and eventually got a solution, for anyone who needs this in the future:

-Open Finder, and find "Go" in the options in the top left bar on the screen.

-While "Go" is open, hold down your option key and "Library" should appear; click on it.

-Click on the "Mail" folder, and then the "V2" folder, and then the "MailData" folder.

-Find all files that start with "Envelope Index" and move them to the Trash folder.

You then should be able to click on Mac Mail, set up your accounts again, and be on your way!