Had a bit of an issue ...
I'm using Kopano. Everything was working fine until I installed the Greylisting addon to postfix to try and restrict the amount of garbage coming though.
All works fine when sending from a machine connected to my local network - except for iPhone/Exchange, which kept generating kopano-spooler errors saying 'Recipient address rejected: Greylisting for ...' etc., and 'No valid recipients' .. which is odd because the recipient in question was actually my gmail account (that I use as a fail-safe measure).
I only see(saw) the problem from the iPhone using an Exchange account - a setup that worked quite happily until I added the Greylisting. Stop Geylisting and it all kicks into life again.
Fixed it by adding 'localhost' to 'postgrey_whitelist_clients.local'.
Hope that won't have any knock on effects ...?