Basically your zimbraMailPurgeSleepInterval needs to be set.
You can do it manually see [SOLVED] junk mail cleanup (probably good to read that anyways) or just upgrade to 5.0.5 and the attribute will get updated to the new default of 1m.
Hence why I said:forum profile is up to date
5.0.5 is Released!
-Don't forget a backup!
I will say that's a very nice script though that can be applied to other folders!
There's also this RFE open: Bug 13435 - User-definable expiration policy for systems folders