After much fighting with XMail, and repeated failure I fell back on the MTA I know best (Postfix), and the local delivery and authentication I know least (mostly because prior to installing XMail on the old server I never had to bother with it.)
So, after checking around I found a fairly well-documented Postfix + Courier-IMAP + SASL + MySQL How-To for Gentoo. While it is specefic in some regards to Gentoo, the majority of the instructions should be fairly straightfoward to transfer to other *nixes.
Anyway, mail is working again, at least mostly. I still need to replace some aliases and a distribution list, but that’s all pretty simple normally, and with the MySQL db added in the mix it gets even easier, since Postfix is looking to the db for virtual domains, users, aliases, relocated mappings, everything. How much simpler can it get?
Now, if there were just one package that combined all the mail functions (kind of like XMail) and integrated with MySQL and came with decent documentation and installed from one package … Maybe Courier will move beyond version 0.53 someday and become more of a player.