Q: Mail not working

Help here >   If Mail on your Mac keeps asking for your password - Apple Support

From the Mail menu bar, please select

          Window ▹ Connection Doctor

Click the Show Detail button. A drawer opens. Click Check Again and post the text that appears.

Anonymize any private information before posting. That means names, email addresses, contents of messages, and anything else that you don't want to make public.

Delete the password from Keychain Access (Applications/Utilities).

Keychain Issues - Resolve  see post by Kappy