You may not be able to fix it without an Account with them, I think it's this...
Mac's Mail App is not as modern as iOS's Mail app, Many eMail providers now require a separate App Password different from your eMail password…
Then you may need Ways to securely access AOL Mail…
https://help.aol.com/articles/allow-apps-that-use-less-secure-sign-in
Or it may just need an App Password separate from the gMail password…
https://support.google.com/mail/answer/185833?hl=en-GB
Apple Mail won't sign in to Google account - Apple Community
Note from PrestoWebDesign…
https://discussions.apple.com/thread/253701684?answerId=257151754022#257151754022
Trying to add Gmail account to Apple Mail… - Apple Community
Yahoo has gone to this too...
https://help.yahoo.com/kb/access-yahoo-mail-third-party-apps-sln15241.html
Yahoo App password…
Generate and manage third-party app passwords
Some older, third-party email apps (that do not use our Yahoo branded sign in page) require you to enter a single password for login credentials. To access your Yahoo Mail account on these apps, you'll need to generate and use an app password. An app password is a long, randomly generated code that gives a non-Yahoo app permission to access your Yahoo account. You’ll only need to provide this code once to sign in to your third-party email app.
https://help.yahoo.com/kb/access-yahoo-mail-third-party-apps-sln15241.html
Using app passwords with apps that don't support two-step verification (microsoft.com)
What about application passwords? Will that keep working?
If application passwords are being used for MFA (Multi Factor Authentication) along with basic auth as another auth mechanism, then when basic authentication is disabled, app passwords used for MFA will stop working too.
Where can I find more information?
There are several resources that we wanted to provide here as additional reading:
https://techcommunity.microsoft.com/t5/exchange-team-blog/exchange-online-email-applications-stopped-signing-in-or-keep/ba-p/3641943
Exchange Online email applications stopped signing in, or keep asking for passwords? Start here. - Microsoft Community Hub
Use a Web Browser to login to your WebMail as a test.
