See if they've gone the way of others or use one of the biggies themselves...
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...
https://support.google.com/mail/answer/185833?hl=en-GB
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)
If gMail not commenting…
https://support.google.com/accounts/answer/2544838?co=GENIE.Platform%3DDesktop&hl=en
https://myaccount.google.com/permissions
