My server is receiving over 100.000 messages per day, 99% of it being spam and 98% of it being sent to invalid addresses (correct domain, but wrong username).
Important note : I have only 1 domain fully configured "" and I've added several domain aliases via the zmprov command ('zmprov cd zimbraDomainType alias zimbraMailCatchAllAddress zimbraMailCatchAllForwardingAddress')

I've followed recipies in wiki and forum threads, so I end with having set 'smtpd_reject_unlisted_recipient' to 'yes' in
Now when I send to I do have a "550 <>: Recipient address rejected:"
but if I send to, I have a "250 Ok"
so it's not good...

It probably has to do with the way the alias has been created..
Is there any workaround to reject unlisted recipients in all aliases domains ?