Upgrade to High Sierra killed Mail /Optus Login

Since ‘upgrading’ to High Sierra three of our four Optus a/c passwords are not working. Is there a simple solution, please, or do I have to spend 3 hrs on the phone to Optus? I can’t find anything on-line.

Apple a/c and password still work.

Any help much appreciated.

Can you log in via webmail?

That would prove whether it’s an account issue or a a Mac OS/ Mail issue.