Sweet, just what I was looking for.... I had already changed my new registration validation messages to say "do not reply to this email, use the Contact Us form located at xxxxx.xxx" because I wanted to use either a fake email address (or one I don't check regularly) for registrations, so spammers registering would not get a valid email address in the "from" field to further spam with.
Then I realized the default Contact Us form used the same address. This completes the puzzle.

* installed *
ver 3.6.2