I have a odd problem with one of my mailservers (running Postfix 3.2.4 on Cent OS 7, from ghettoforge repo). Postfix seems not to be able to lookup hostnames (note, this is a fresh setup). DNS resolution works on the host. I triple checked thr PTR-,MX- and A-Records for this server, and they are correct. But whenever I try to send a mail to the server it gets rejected:
NOQUEUE: reject: RCPT from unknown[x.x.x.x]: 450 4.7.25 Client host rejected: cannot find your hostname, [x.x.x.x]; from=<foobar> to=<[email protected]> proto=ESMTP helo=<backupmx.mydomain.com>
In this case, the backup mailserver tries to send over its mail to the freshly setup main mailserver. In the helo field the hostname is stated correctly. Postfix just seems to not resolve it.
I also tried to send mail to this server via one of my public mailaccounts and the error is the same.
Any idea what could be the problem here? (Note: yes, I could just remove stuff like reject_unknown_client_hostname or reject_unknown_sender_domain, but I want to avoid spam. (But, when I remove the two I mentioned I can receive mail again))
Thanks for helping me.
Update:
I decided to remove the two directives to get on with deeper troubleshooting. and it gets worse. Receiving mail works now, but sending is something else. When I try to send a mail to one of my gmail accounts, postfix throws this:
NOQUEUE: reject: RCPT from unknown[x.x.x.x]: 450 4.4.4 <[email protected]>: Recipient address rejected: Unable to look up mail exchanger host gmail-smtp-in.l.google.com: Device or resource busy; from=<[email protected]> to=<[email protected]> proto=ESMTP helo=<somehost>
So postfix definitely is not able to lookup hostnames? I am a little puzzled by this.
