This is a memory aid, like a lot of these posts. Because I forget how to do things and have to figure it out again.
Adding mail accounts on a virtual mailbox domains requires two things: a) make an entry in /etc/postfix/virtualmb and posthash it, b) ensure the username exists in the users database on tarragon.
Adding mail accounts on the primary wmbuck.net domain (without creating a login account and home directory, etc.) requires a) make an entry in /etc/postfix/localrecipientsmap and posthash it, and b) adding the username to the users database.