I just received a note from Adam Engst informing me that my Address Book "has gotten whacked and is sending messages to firstname.lastname@example.org instead of email@example.com." That seemed like an odd mistake to make, since the "sponsors" address (which he uses to prioritize communication with our sponsors and potential advertisers) doesn't even appear in my Address Book database.
The actual culprit is Apple Mail's Previous Recipients list, which stores recent email addresses for later auto-completion when you start typing someone's name or address in a recipient field. In this case, I'd recently received a message from Adam when he was using firstname.lastname@example.org, which was added to the list. When I typed "Adam" in the To field of an outgoing message, Mail auto-completed the entry as "Adam Engst, email@example.com". I was typing quickly and didn't notice the address before moving on to the next name - hence the puzzled reply from Adam.
To work around the problem, you can remove the address from the Previous Recipients list in one of two ways:
This removal feature is designed so that if you mistyped someone's information at one point, you can remove that erroneous address so you don't trip over it again.
However, those steps fix the problem only temporarily; the next time Adam includes me on a message from the sponsors address, it will again be added to my Previous Recipients list.
Also, there was another oddity: I would have thought that Adam's regular address would appear first in any list, since I use it far more than the other address, but that wasn't the case. Instead, the sponsors address appeared first because the name associated with it was "Adam Engst", while the entry in my Address Book database is "Adam C. Engst". Apparently, a name with no middle name is alphabetized higher than one with a middle name, which also gave me the clue as to how to fix the problem permanently.
Instead of removing the sponsors address, I chose Add to Address Book (available in either of the two methods mentioned above) and changed the name to "TidBITS Sponsorship Program" (to make sure I really don't stumble on it later). Now, Adam's main address appears first in the list. I can press the comma, right-arrow, or Tab keys to lock in the correct address and move on to the next field without wondering if I've misdirected the mail.