Had same problem with Linux Mint 19 (based on Ubuntu 18.04), initially on Comcast pop mail about a week ago. Comcast customer service told me to switch to imap and that worked intially, but the problem occured again so I now couldn't receive Comcast emails on imap a few days later.
Checked other forums and found temporary solution per this thread: https://ubuntuforums.org/showthread.php?t=2406399
I had to accept a Comcast security certifcate that wasn't considered valid by Thunderbird as an exception, but the fix has continued to work for three days. Before doing so, I also tested another e-mail program (Gerry), but it had the same problem connecting to Comcast mail service over SSL/TSL. Comcast emails were getting through on imap in Windows 10 Mail, so the problem with incompatable security certificates seems to only be present when on Ubuntu based distros.
Comcast customer support on this issue has been pitiful! Useless suggestions from their reps and even now, several days after I reported the problem(s), I still haven't been contacted by the higher e-mail fixit group that they supposedly escalated it to . If it wasn't for forums like this and google search, problems like this would never get resolved. Thanks to the other users who have contributed to solving the problem while Comcast does nothing!
