There is a possibility that your SMTP server settings may be incorrect, causing the error.

I haven't been able to get it to work. I could not get Comcast tech support on the phone. I went to the Xfinity store. They didn't know how to fix it. But they claimed that Xfinity/Comcast is no longer allowing third-party email programs.

I don't know if that is true or not. When people don't know how to do something, they will tell lies like this.

My wife's email IS still working.

Does anyone know about Comcast/Xfinity and third-party email programs?

That cannot be true or we would be inundated with help requests. Maybe if you post screenshot of your SMTP server settings, someone here can offer help, as comcast users are regularly posting here. Thank you,

This link has some info of possible use. https://www.xfinity.com/support/articles/third-party-email-access

The attachment may help as well:

"This link has some info of possible use. " - this seems to have worked. It said to check "Third party access security". It was already checked. I unchecked it and then checked it again, and did two tests, and it is working!

I need to use it some more, but this probably solved the problem.