I have setup my Mac Mail to get emails from my Comcast account and it has been working for some time. Recently, it keeps asking for my password. After some debugging, I found out the problem came from a Mac Mail update. Here are the steps to fix the problem.
On Mac Mail, select
Mail | Preferences... | <mail account> | Advanced
Then uncheck option
[] Automatically detect and maintain account settings

Let me know if this is helpful.
Poon
4/17/2016