Exchange account in Mail has wrong username, but is not editable
I've been looking for an answer to this as well. Here's how i resolved it.

I found the file that stores the username in ~/Library/Accounts/Accounts3.sqlite. I used the sqlitebrowser app from http://sqlitebrowser.org/ to open it.


Drag and drop Accounts3.sqlite to the dock icon to open the file.
Click the Browse Data tab.
Change the Table to ZACCOUNT.
I used the ZACCOUNTDESCRIPTION field to determine the right row to make the change.
Update the value in the ZUSERNAME field.


For me, entering the full email address didn't work. I used domain\user instead.