Making MacMail work on Comcast
I had same issue on my Mac Mail with three separate Comcast email accounts. My 5 other email accounts all work fine.
Try this: Go to Mail Preferences - Accounts - (select your Comcast account) - Advanced tab. Now change your Port to 995. Comcast told me to change it to 587, but that didn't work. I called Comcast two more times and got someone who said to change it to 995.
I don't believe this would make a difference, but (for the record) I have the SSL box checked and am using Password Authentication.
I will tell you that it took about 10 minutes before my Comcast mail came in on one of my accounts and only about a minute for the two other lesser-used accounts.
Good luck! 🙄