Mate, sorry for taking so long to get back to you. If you don't mind re-entering your account details, then find a file named com.apple.mail.plist in your Preferences folder and drag it to the Desktop whilst Mail is closed. Reopen Mail and re-enter your account details and customised settings. Before you do this, make an Archive of the Mail folder in your Library and keep it on the Desktop as backup. 🙂