Error: Could not connect to this POP/IMAP server. Check your network connection and that you entered the correct information in the Account preferences.
Error:
Unable to Receive Mail
The mail server denied access to the account "______" because an administrator or other mail client was using it when Mail tried to log in. Try again later.
The server returned an error: account is locked by another session or for maintenance, try again.
Connection Doctor is a diagnostic tool located in Apple's Mail application. It will test the connection to all enabled accounts in Mail and display a success message or an error.
The Connection Doctor is located under the Window menu in Mail.