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Our postfix sometimes rejects emails. I see this error in our postfix log (/var/log/mail.log):

Feb  9 16:16:33 sd-15xxx postfix/smtpd[1972]: NOQUEUE: reject: RCPT from unknown[x.x.x.x]: 450 4.7.1 Client host rejected: cannot find your hostname, [x.x.x.x]; from=<> to=<> proto=ESMTP helo=<>

Here is my postconf -n :

append_dot_mydomain = yes
biff = no
config_directory = /etc/postfix
disable_vrfy_command = yes
inet_interfaces = all
mydestination = localhost, localhost.localdomain,
mydomain =
myhostname =
mynetworks = y.y.y.y <our server IP address>
relayhost = 
smtpd_banner = $myhostname ESMTP (Debian/GNU)
smtpd_client_restrictions = reject_unknown_client,        permit_mynetworks
smtpd_helo_required = yes
smtpd_recipient_restrictions = permit_mynetworks,        reject_unauth_destination,        reject_unknown_recipient_domain,        reject_non_fqdn_recipient
smtpd_sender_restrictions = permit_mynetworks,        reject_unknown_sender_domain,        warn_if_reject reject_unverified_sender
virtual_alias_maps = mysql:/etc/postfix/,mysql:/etc/postfix/
virtual_gid_maps = static:5000
virtual_mailbox_base = /var/spool/vmail/
virtual_mailbox_domains = mysql:/etc/postfix/
virtual_mailbox_maps = mysql:/etc/postfix/
virtual_uid_maps = static:5000

It doesn't occur often, but it could have serious consequences if we missed an important email.

Do you see where this problem comes from? Is it a problem of configuration or a reverse DNS problem (as I read somewhere)?

Here is the result of nslookup x.x.x.x :  name =

Authoritative answers can be found from: nameserver = nameserver = nameserver =  internet address =  internet address =  internet address =

and the result of dig x.x.x.x :

; <<>> DiG 9.5.1-P3 <<>> x.x.x.x
;; global options:  printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 5920
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 0

;x.x.x.x.           IN  A

.           10800   IN  SOA 2010020901 1800 900 604800 86400

;; Query time: 227 msec
;; WHEN: Wed Feb 10 13:45:54 2010
;; MSG SIZE  rcvd: 106

I don't even know if the problem comes form our side or from the sender side.

Thanks for your help.

share|improve this question
Providing un-anonymized output would really help to determine the issue. If you feel like you can do that, please do. – Bill Weiss Feb 9 '10 at 19:59
up vote 6 down vote accepted

This is caused by the reject_unknown_client instruction ( )

share|improve this answer
So I should comment the 'reject_unknown_client' in smtpd_client_restrictions of /etc/postix/, that's it? But why isn't it a sender restriction? – Flackou Feb 10 '10 at 13:00
Ok, I commented it and the email who was being rejected continuously has been accepted. I hope this security restriction wasn't too sensitive. – Flackou Feb 10 '10 at 13:17
How about when the reverse DNS lookup clearly is returning the correct response (that is ANSWER SECTION), what could be the reason? – mfx May 20 '11 at 19:53

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.

share|improve this answer
nslookup gives : Non-authoritative answer: name = Authoritative answers can be found from: nameserver = nameserver = nameserver = internet address = 65.57.a.b internet address = 63.211.d.e internet address = 64.154.f.g Is it ok? – Flackou Feb 9 '10 at 18:01
Can you show the output of "dig x.x.x.x"? Edit your question and put it there instead of as a comment so that formatting can be retained. – Bill Weiss Feb 9 '10 at 18:50

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