A couple weeks ago I could not logon to my Comcast email, kept trying and finally suspected I had been locked out because of repetitive attempts. I changed the password, made sure I used all the current settings, and was able to logon. Then yesterday morning, when I tried sending email, I got the warning "Cannot send message using the server Comcast" with all outgoing emails. It has persisted, and I again changed my password to get on the forum. Then I entered my new password into to email client settings, but it failed to go online. I'm using POP in Mail in macos 10.13.6. Has xFinity just made any changes re the outgoing servers?
Try deleting the account from Mac Mail, and then recreate it from scratch. Let Mail auto-configure the account for you. See if that solves the issue.
Thanks. I finally got things fixed by going through each account and re-entering settings manually - don't understand how they had changed. As far as letting Mail set up and configure an account , that only works for IMAP and I'm old-school and trying to stay om POP for now.