Safari is one of the browsers supported by Comcast, and has been for a long time. Do you have another browser you can try with to see if the same thing happens?
I have tried it on the chrome browser and it isn't as bad but still get the error messages just not as often. It also take a very long time for the emails to send. My spouse is using a microsoft laptop and has no issues at all sitting right next to me. This is very frustrating.
I have had the same Server Unavailable problem for over a month. We use Safari in the latest update of MacOS 10.15 Mojave. If I try to send, it works after several attempts. Also happens in Firefox, but much less frequently. Have emptied cache and history. Same problems at other wifi locations, friends' houses. Problem when sending via my university email system.
On all 4 Macs, we get messages all the time like “Server Unavailable,” and sometimes “secure connection failed,” “The page you are trying to view cannot be shown because the authenticity of the received data could not be verified,” and rarely “An error occurred during a connection to s0.2mdn.net. PR_END_OF_FILE_ERROR.” Often, I have to wait while my Comcast email loads, and have to click or refresh multiple times or try to log in again. I have rebooted the modem, router and computers a number of times to no effect.
I'm in Westerna PA and no problem using Safari to get into Xfinity Connect. If you are not using the most up to date version of Mac OS try to update to Catalina 10.15.6 and use an anti male ware software like MalewareBytes to make sure your computer is secure. If you know your email password can you set up your email on your Mac using the native email?
Mojave is less than 2 years old. It should still work! I never had any issues with comcast email until about 6 to 8 weeks ago when everything went south. Never. Comcast did something with their email server software or something related at that time; resulting in numerous problems with Macs. Comcast should have the resources to identify and fix it. These complaints should be forwarded to the Comcast IT employees that made that change to the software 8 weeks ago. The cause (and fix) should not be hard to identify if the right Comcast or contract workers are given the information.