Mac Mail Yosemite and Comcast.net
Still on Yosemite. Comcast mail through mac mail stopped working a week ago. Is this the end for Yosemite and mac mail?
Then you are already using modern TLS encryption, introduced with Safari 9. That should be perfectly fine. No need to upgrade your MacOS to improve encryption, that is NOT obsolete.
What is more likely is that Comcast made some subtle change in the way they want you authenticate to pick up and/or send your mail.
To check what are the commonly expected settings, enter a real (or imaginary) user-name@comcast.net or whatever you are supposed to be using into the lookup on this web page:
when you click (verify) button, it will lookup the generally accepted settings.
Mail > Window > Connection doctor ...
will take you to a status page. click any red light to go directly for the settings for that server.
compare what you set to what is generally accepted.