If I remove the email address that is unable to send or receive emails, I think I will have to back up the pst and dat files. I think when deleting an account in Outlook it will delete everything. Including the pst and dat files. I thought thunderbird was just a browser. Or does Thunderbird function like Outlook?

Kevin,

This could be an issue with a profile corruption in Outlook, but it might not be.

If you are comfortable with setting up another email client such as Thunderbird and adding all the accounts there, and seeing if they load messages correctly, that would be a useful diagnostic step to see if the issue lies with Outlook or Spectrum (it wouldn't be the first time an email service provider says, "we're having no issues," when the issue lies with them).  If everything loads in Thunderbird, then it at least could be an issue with your Outlook profile.

Also, you should let Outlook do the setup for the servers for Spectrum itself.  I know that they have accurate data about which IMAP and SMTP servers, ports, and protocols are used by Spectrum.  You could try re-adding the account and do NOT manually set up anything.  Just give Outlook the email address and password when prompted and let it do the rest.
--

Brian - Windows 10, 64-Bit, Version 21H2, Build 19044  

Here is a test to find out whether your mission in life is complete.  If you’re alive, it isn’t.
     ~ Lauren Bacall