Postfix is rejecting some incoming emails ("Client host rejected: cannot find your hostname")

This is caused by the reject_unknown_client instruction ( http://www.postfix.org/postconf.5.html#reject_unknown_client_hostname )

That's because the ip address sending the email (x.x.x.x in your case) isn't returning a valid reverse dns entry - either because there isn't a ptr record on their name server giving a valid hostname or because your dns lookups are failing. The first thing to try is 'nslookup x.x.x.x' to see what you get.