postfix

[A.G.M. Velthof]

Hello, in my maillog (postfix) I see:
- RCPT from unknown client host rejected: cannot find your hostname
and
- warning xxx.xxx.xxx.xxx address not listed for hostname

Can anyone tell me what causes those messages?
(smtpd_client_restrictions or smtpd_sender_restrictions)?

[Matti_Kurkela]

I believe it's smtpd_helo_restrictions ->reject_unknown_helo_hostname or smtpd_client_restrictions -> reject_unknown_reverse_client_hostname or reject_unknown_client_hostname.

If you have smtpd_delay_reject set to yes (=default), any rejection messags about the HELO/EHLO phase are delayed until the client gives the RCPT command. The documentation at www.postfix.org says: "This feature is turned on by default because some clients apparently mis-behave when the Postfix SMTP server rejects commands before RCPT TO."

The client has claimed some hostname in its HELO/EHLO command, but your Postfix is making a reverse DNS lookup ("what is the name of this IP address?") and not getting the same name in answer, maybe not getting any name at all. This looks like the client is trying to lie about its name, so postfix is rejecting it.

MK

[A.G.M. Velthof]

So what can be the problem when it turn the delay off.
Will I get a problem on my postfix mailserver?