Thnx for your reply. I've been gone for several days, so that's why I didn't respond earlier.
I've been using Zarafa for quite some time, also with another phone (SE Xperia X1) and Push-mail works perfectly there. And it's been working fine for several days on the Hero too, but all of a sudden it stopped. I can make it work for up to an hour (at very most) by setting the download frequency to 'on arrival' which also proofs the server-side should be OK.
I'm not getting any errormessages, not on the phone nor on the server.
I've tried all things you mentioned, except a hard-reset. I'll try that later today as I want to make a backup first.
The funny thing is that I saw somebody else at this forum having the same symptoms, only with a real MS Exchange-server. Sadly enough, I can't find that post anymore...