Comcast email not working with Apple Mail

My comcast email has automatically fed into my Apple mail for years. It stopped two days ago and after 2 hours on the phone with Comcast and 1.5 with Apple it still doesn’t work. I have clicked the 3rd party OK button on Comcast (it was already selected), rebooted, changed my password, and done about 500 checks with different options and nothing works. I was able to add a gmail account in two seconds just as a test. I have six email addresses sending info to my Apple mailbox. I would love any ideas you may have. Interestingly, I am still receiving Comcast emails, I just can’t send them back out. The same thing is happening on my I-phone and I-pad.