I've had (and still have) the same problem on my iPod Touch 6th generation for several weeks now. The errors have been changing but it's all the same problem. The first errors said the password was wrong for SMTP. I went into Settings and re-typed the password a thousand times - no help. I tried on another day and it said my IMAP password was invalid. Again I typed in the correct password over & over (I know it's correct because it works on the Comcast website).
Today I just tried again - retyping the password into IMAP & SMTP and right now it says "Cannot Get Mail. The mail server "mail.comcast.net" is not responding. Verify that you have entered the correct account info in Mail settings."
I even checked the port settings against what I have on my iPad Mini, which is still working for Comcast email - the ports are the same.
And... I don't know if this is related or not, but maybe it is - on my MacBook Pro a few minutes ago, I clicked Quit Mail. When I opened Mail again, I'm now getting asked for my Comcast password and when I put it in it says it's an unverified email address or password. I tried to fix it in Settings, Internet Accounts but I'm getting the same error there too.
So maybe the problem happens when you quit Mail - not sure. On the iPod Touch though I just hadn't used the Mail function for a long time.