If this message persists, you can learn how to resolve sync issues in Mail and Calendar apps. During this time, you may see 'Not synced yet' in the message pane. During attempts to resolve the problem of my existing Comcast account refusing to connect and not recognizing the new password, I deleted the account in the mail tab and tried reinstalling which just made things worse.Note: It may take a few minutes for your account to synchronize.

To try to attempt to get this to work I removed 2FV but still no luck. The problem started when I added two factor verification and changed the password of my Comcast account. I have a hunch I’ve lost the ability to access Comcast email via Apple Mail forever. Not extremely helpful but a probable indicator of some corruption somewhere. But I do remember in one attempt at fixing this while attempting to add it via the “system preference” “internet account” tab instead of the mail tab that I got an error that said “system preferences error”. I was blaming bad handshaking during the verification of account data thinking Comcast did not keep up with the latest operating system. Now I am not familiar enough with keychain to look for problems there but you might be on to something as I am able to set the account up without problems on one Mac running Yosemite but I can’t on the Mac running High Sierra. There is no opportunity to do any settings as you suggest that I know of. The screen then says “verifying” and then says “unable to verify ID / password”. After using “add account” in the mail tab, selecting “mail account” one has to enter account and password info. One problem is that I can’t even get to a state where I can enter receiving port and outgoing port information manually.

Setting up mac mail with comcast