I seldom use my comcast.net (now xfinity.com) email address. I prefer to use my Gmail account and my account at my employer but I have been importing messages sent to my comcast.net/infinity.com email address into my Gmail inbox using POP3. A few weeks ago, this feature stopped working due to authentication failures so I edited my external account information in Gmail for my xfinity.com email account to make sure that Gmail is using the correct password to try to fetch my email. This attempted solution did not work. Gmail still cannot authenticate connecting to xfinity.com to fetch my email via POP3 using my xfinity.com password. For now, I have simply deleted my infinity.com account from my list of external email addresses at Gmail. I won't miss it since I have not read anything of importance sent to my email address at xfinity.com/comcast.net in the last few years. But for infinity.com users with Gmail accounts who actively use their xfinity.com account, this failure looks important.
I can use that Gmail feature with my Comcast account just fine. Try re-adding it from scratch. POP3 should work with Comcast accounts, although thy are IMAP compliant by default. If you still can't get it to work right, you can always go into the Comcast email account settings and just have it auto-forward everything to the Gmail account.