Postfix refuses incoming mail from some (good) servers with 4.7.25 but they have correct reverse DNS

Question

Scenario:

This is a production mailserver since 4 years, with around 2000 mailboxes on ~50 domains, and works good with almost any other mailserver. Logs are constantly watched and the following issue arised recently.

Edit 1: Ubutntu 16.04.6 - postfix (3.1.0-3ubuntu0.3)

Issue:

There is a mailserver who seems unable to deliver mail to this system.

It fails with 4.7.25 Client host rejected.

NOQUEUE: reject: RCPT from unknown[159.135.224.3]: 450 4.7.25 Client host rejected: cannot find your hostname, [159.135.224.3]

That's how I configured postfix as I want to avoid relayers without reverse dns record.

root@mail:~# grep restrictions /etc/postfix/main.cf
smtpd_relay_restrictions = permit_mynetworks permit_sasl_authenticated defer_unauth_destination
smtpd_recipient_restrictions = permit_mynetworks, permit_sasl_authenticated, reject_unauth_destination, reject_rbl_client zen.spamhaus.org, check_recipient_access mysql:/etc/postfix/mysql-virtual_recipient.cf, check_recipient_access mysql:/etc/postfix/mysql-virtual_policy_greylist.cf, check_policy_service unix:private/policy-spf
smtpd_helo_restrictions = permit_sasl_authenticated, permit_mynetworks, check_helo_access regexp:/etc/postfix/helo_access, reject_invalid_hostname, reject_non_fqdn_hostname, reject_invalid_helo_hostname, reject_unknown_helo_hostname, check_helo_access regexp:/etc/postfix/blacklist_helo
smtpd_sender_restrictions = check_sender_access regexp:/etc/postfix/tag_as_originating.re , permit_mynetworks, permit_sasl_authenticated, check_sender_access mysql:/etc/postfix/mysql-virtual_sender.cf, check_sender_access regexp:/etc/postfix/tag_as_foreign.re, reject_unknown_sender_domain, reject_unknown_reverse_client_hostname, check_client_access hash:/etc/postfix/client_access, reject_unknown_client_hostname
smtpd_client_restrictions = check_client_access mysql:/etc/postfix/mysql-virtual_client.cf

BUT it has a valid one, test exectued locally on mailserver:

DNS tests on 159.135.224.3

root@mail:~# nslookup 159.135.224.3
Server:         1.1.1.1
Address:        1.1.1.1#53

Non-authoritative answer:
3.224.135.159.in-addr.arpa      name = relay.teamgioia.it.

Authoritative answers can be found from:

root@mail:~# dig 159.135.224.3

; <<>> DiG 9.10.3-P4-Ubuntu <<>> 159.135.224.3
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 23656
;; flags: qr rd ra ad; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 1452
;; QUESTION SECTION:
;159.135.224.3.                 IN      A

;; AUTHORITY SECTION:
.                       10165   IN      SOA     a.root-servers.net. nstld.verisign-grs.com. 2019083000 1800 900 604800 86400

;; Query time: 6 msec
;; SERVER: 1.1.1.1#53(1.1.1.1)
;; WHEN: Fri Aug 30 12:18:21 CEST 2019
;; MSG SIZE  rcvd: 117

In resolv.conf:

nameserver 1.1.1.1
nameserver 1.0.0.1

Questions:

Why does reject_unknown_client_hostname isn't working as I expect?

If that's not this server's fault but sender's fault, How can I whitelist some senders to avoid their mail to be rejected?

Could you also explain or speculate what's their fault?

Answer 1

The IP addresses don't match the domain lookup.

Quote from the documentation:

reject_unknown_client_hostname
Reject the request when 1) the client IP address->name mapping fails, or 2) the name->address mapping fails, or 3) the name->address mapping does not match the client IP address.

If you look up the client IP address, then look up the resolved DNS name again and compare those:

$ nslookup 159.135.224.3
3.224.135.159.in-addr.arpa      name = relay.teamgioia.it.
$ nslookup relay.teamgioia.it.
Non-authoritative answer:
relay.teamgioia.it      canonical name = mailgun.org.
Name:   mailgun.org
Address: 52.2.180.207
Name:   mailgun.org
Address: 52.4.250.90

You see, that the original IP address does not match the DNS resolved addresses. That's why it fails the check.