I have a server running Postfix (3.1.0-3ubuntu0.3) that has been in
production for a few months and is generally working well.  However I've
been struggling to resolve an issue where some legitimate servers cannot
send inbound mail, where there appears to be an issue with the sending
server's DNS.

Here is an example:

Mar  5 23:25:47 enceladus postfix/postscreen[5155]: CONNECT from
[]:51876 to [x.x.x.x]:25
Mar  5 23:25:47 enceladus postfix/postscreen[5155]: PASS OLD
Mar  5 23:25:47 enceladus postfix/smtpd[5159]: warning: hostname
104-37-111-105.static.dbsintl.net does not resolve to address Name or service not known
Mar  5 23:25:47 enceladus postfix/smtpd[5159]: connect from
Mar  5 23:25:47 enceladus postfix/smtpd[5159]: disconnect from
unknown[] ehlo=1 mail=0/1 quit=1 commands=2/3

Details on the sending host:

dig 104-37-111-105.static.dbsintl.net

; <<>> DiG 9.10.3-P4-Ubuntu <<>> 104-37-111-105.static.dbsintl.net
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 53175
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 1

; EDNS: version: 0, flags:; udp: 4096
;104-37-111-105.static.dbsintl.net. IN  A

dbsintl.net.            60      IN      SOA     ns1.ral.hostedsolutions.com.
hostmaster.hostedsolutions.com. 2013061720 10800 3600 604800 604800

;; Query time: 19 msec
;; WHEN: Tue Mar 06 17:54:38 UTC 2018
;; MSG SIZE  rcvd: 136

My smptd recipient restrictions from main.cf:

smtpd_recipient_restrictions = check_policy_service unix:private/policy-spf,
reject_unknown_recipient_domain, reject_unauth_pipelining,
permit_mynetworks, permit_sasl_authenticated, reject_unauth_destination

I previously had included reject_unknown_client_hostname, but removing it
seems to have no effect.  That's about the only seemingly relevant thing
I've found online, and I'm really at a loss as to how to proceed.

Also interesting, when attempting to register for the Postfix forum, I never
received the confirmation e-mail and had to use a backup account.  I have
seen this behavior once or twice before, where the remote server connects
and then disconnects without appearing to do anything, but have no idea
where to begin troubleshooting it:

Mar  6 17:37:13 enceladus postfix/postscreen[12658]: CONNECT from
[]:53413 to [x.x.x.x]:25
Mar  6 17:37:19 enceladus postfix/postscreen[12658]: PASS NEW
Mar  6 17:37:19 enceladus postfix/smtpd[12668]: connect from
Mar  6 17:37:19 enceladus postfix/smtpd[12668]: disconnect from
n5.nabble.com[] ehlo=1 mail=0/1 rcpt=0/1 data=0/1 rset=0/1
quit=1 commands=2/6

Please let me know what additional information needed to dig deeper into
these issues.  Thank you!

Sent from: http://postfix.1071664.n5.nabble.com/Postfix-Users-f2.html

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