Was about to send a question to a dev I've never talked with before. Decided I should probably use the same 'donotspam addy I did when I downloaded his app and that he used to send me a message.
Alas, when I attempted to select that addy as my From list, it was not available. It's actually just an additional (alias) addy in my comcast account, but I thought I could 'fake' Mail into accepting a 'new' account with the info I had available...
First off, I noticed Mail decided to change my comcast email address, in the already in-use account to the 'donotspam' addy.
OK, Mail wants the two addy's to be the same... but the main comcast addy is what should stay in the main account, I think. And I managed to get that changed back. I also changed the account name to help convince Mail to save my entries...
Next, I see that the 'new' account has now changed the info there to attempt to match the main comcast account...
OK, let's just back up a bit. In fact, let's just delete this new 'donotspam' account in Mail and forget this whole scheme. Clicking the "-" (Delete/Remove/Kill/Ignore) mini-button does nothing helpful! Instead, it brings up a dialog reminding me that Mail and Internet Accounts System Prefs are now joined at the hip!
About this time I get the "What is the password for that new email account?" dialog! Remember the endless-loop that thing has done?!
I gave it a password and it went away, at least long enough to get to the System Prefs.
Now I see that the Prefs window has the two comcast accounts, but when I hit the delete button with what claims to be the 'donotspam' addy, I get a warning asking if I really want to delete the main comcast account addy! "Uh... not really, why not delete the one that you have labeled, instead?" Unfortunately, I got no answer. I guess Siri is not running on my iMac...
Long story short, I now have Mail running, with the correct info for the my four accounts/domains. OTOH, "Internet Accounts" still shows the main comcast account with an addy of 'donotspam'.
Who knows how long before I have to disable/delete/remove/erase/block-out/etc. the whole comcast set up and start over?