I have solved this!

Cyrus has a built-in filtering system which can redirect mail to folders according to rules you supply. I found on my old server I had set up some rules, but had forgotten all about it.

The rule I needed (for user nikki) was:



And, when logged in as user nikki, I can add this rule to the system using sieveshell as follows (nb: the sieveshell prompt is >:



Of course, this presupposes you can open a shell for each user (I logged in as root and used ), and that you know each user's password (which I do).