Hello,
Is there any way to reject mails from domains with useless MX records?
I want to reject cases where the MX records themselves are syntactically
valid, but the name they point to doesn't have any A or AAAA records.
I kinda expected reject_unknown_sender_domain to already do that, but
the docs actually say:
Reject the request when Postfix is not the final destination for the
sender address, and the MAIL FROM domain has 1) no DNS MX and no DNS A
record, or 2) a malformed MX record such as a record with a zero-length
MX hostname (Postfix version 2.3 and later).
So I suppose since the MX record itself is fine, although useless,
reject_unknown_sender_domain is happy with it.
See this log line for example:
2025-10-20T00:27:12.626227+02:00 mail postfix/mx/smtpd[226824]: warning:
Unable to look up MX host irondelle.o2switch.net for Sender address
[email protected]: Name or service not known
So the domain has an MX record:
; <<>> DiG 9.18.33-1~deb12u2-Debian <<>> easydoli.fr in mx
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 49146
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 1232
;; QUESTION SECTION:
;easydoli.fr. IN MX
;; ANSWER SECTION:
easydoli.fr. 3255 IN MX 1 irondelle.o2switch.net.
But that MX doesn't have any A records:
; <<>> DiG 9.18.33-1~deb12u2-Debian <<>> irondelle.o2switch.net. in a
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 41049
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 1
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 1232
;; QUESTION SECTION:
;irondelle.o2switch.net. IN A
And for completeness sake, let's test AAAA as well:
; <<>> DiG 9.18.33-1~deb12u2-Debian <<>> irondelle.o2switch.net. in aaaa
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 15116
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 1
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 1232
;; QUESTION SECTION:
;irondelle.o2switch.net. IN AAAA
Thanks!
Antonin
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