One additional troubleshooting thought / suggestion.....
Make a new user on your troubled Mac so you have a clean slate to start with....then try to access your new email account from that clean slate to see if the issues you are experiencing with your normal user account are still affecting you.
The new user account that you make should isolate any prior caches / plists / add-ons / etc. that might be affecting your normal user logon.....
Let us know how it goes.
