Thank you everyone that pointed out the use of the recipient_delimiter parameter. It took several of you pounding on my thick skull to penetrate it, but I finally looked at the parameter.
This is embarrassing: I have *never* looked at that parameter -- and when the first person mentioned it I thought "where did they come up with that? They don't understand my problem!" -- because I had a [wrong] preconceived notion of what that parameter was for. "Recipient delimiter" to me means commas/semicolons separating multiple recipients in message headers. I never stopped to think that Postfix wouldn't likely care about that, since delivery is determined by the envelope, not the contents, and recipients are clearly specified on individual lines. Duh.... Yes, recipient_delimiter is what I was looking for -- and what I had used years ago to enable the "+" in the first place. All I had to do was open my eyes and look up! :-)
