It took me several hours, but I finally figured out why.
I had configured Postfix to deliver mail into my Cyrus IMAP server. The local accounts are all first name only such as 'tony' but our e-mail addresses use first name and first initial of the last name such as 'tonym@'. Since I'm now using NIS to authenticate users on the network and use just first name, I thought it would be trivial to just set an alias in Postfix to deliver them to the right Cyrus mailboxes.
Boy was I wrong.After turning on debug logging, I was seeing the following: