One thing that quickly became annoying about the way that I'm doing the old-domain-points-to-new thing, is that it seems my mail server or something (MIMEDefang or SpamAssassin, maybe) is rewriting the incoming mail so that I can't tell just from looking at the To: header whether I need to update the sender about the new address or not. There were clues in the other headers, though, that this was taking place.
Procmail to the rescue...
| formail -i "X-message-flag: Probably sent to old domain"
Then, I just have to get Pine to pretend that it's Outlook for a half second and display that header if it's there. Problem solved.