Mail sometimes not forwarded

I have forwarding rules set up for all my users in Usermin (version 1.570 running on Ubuntu 12.04 LTS) to always forward mail to an external address, typically a gmail account. I don’t therefore really need to run Usermin at all (and didn’t for a while), but for complicated reasons (see I’ve been running spamassassin on forwarded mail and that has to be set up in Usermin.

Anyway, the problem is most mail gets forwarded but some goes to the Usermin inbox instead where it gets ignored. I’ve had a close look at the differences between mail that is forwarded and mail that isn’t but so far I haven’t seen anything significant.

I suspect the problem is to do with the exception “Suppress forwarding of bounce messages”. I think turning that off fixes this problem, though it will be a while before I’m sure. The problem is, I’d like it left on (as it is by default) because I’ve had problems with “backscatter” spam in the past. Also I can’t see any reason why the particular messages that don’t get forwarded should be incorrectly classed as bounce messages. I can’t see anything in the procmail configuration that would cause this.

Has anyone else had the same problem and found a solution?