Two days ago my comcast email stopped opening on my Mac computer. It begins to load up then stops. I am using OS X EL CAPTAIN. The last update to the system was installed on Nov 12, 2020 almost a week before the problem began to happen. If I attempt to open the email through comcast.com site, the msg reads unable to establish a secure connection with Safari.
You might want to upgrade to Catalina. I have it, and Safari v14.0.1 and I can get into the email with no issues.