It seems, postcrossing com has a minor (very minor) issue with the DNS servers, it could NOT be the reason for mail.ru to refuse the messages, but the issue is the following:
The postcrossing.com itself is delegated to the following DNS servers:
postcrossing.com. 172800 IN NS ns1.dnsmadeeasy.com.
postcrossing.com. 172800 IN NS ns4.dnsmadeeasy.com.
postcrossing.com. 172800 IN NS ns3.dnsmadeeasy.com.
postcrossing.com. 172800 IN NS ns0.dnsmadeeasy.com.
postcrossing.com. 172800 IN NS ns2.dnsmadeeasy.com.
But in the zone file at these name servers we have another NS records, they don’t match the NS servers the domain is delegated to:
% dig @ns2.dnsmadeeasy.com postcrossing.com ns
; <<>> DiG 9.10.6 <<>> @ns2.dnsmadeeasy.com postcrossing.com ns
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 44696
;; flags: qr aa rd; QUERY: 1, ANSWER: 5, AUTHORITY: 0, ADDITIONAL: 11
;; WARNING: recursion requested but not available
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 1280
;; QUESTION SECTION:
;postcrossing.com. IN NS
;; ANSWER SECTION:
postcrossing.com. 86400 IN NS ns3.postcrossing.com.
postcrossing.com. 86400 IN NS ns4.postcrossing.com.
postcrossing.com. 86400 IN NS ns0.postcrossing.com.
postcrossing.com. 86400 IN NS ns2.postcrossing.com.
postcrossing.com. 86400 IN NS ns1.postcrossing.com.
;; ADDITIONAL SECTION:
ns0.postcrossing.com. 2592000 IN A 208.94.148.2
ns1.postcrossing.com. 2592000 IN A 208.80.124.2
ns2.postcrossing.com. 2592000 IN A 208.80.126.2
ns3.postcrossing.com. 2592000 IN A 208.80.125.2
ns4.postcrossing.com. 2592000 IN A 208.80.127.2
ns0.postcrossing.com. 2592000 IN AAAA 2600:1800::1
ns1.postcrossing.com. 2592000 IN AAAA 2600:1801:1::1
ns2.postcrossing.com. 2592000 IN AAAA 2600:1802:2::1
ns3.postcrossing.com. 2592000 IN AAAA 2600:1801:3::1
ns4.postcrossing.com. 2592000 IN AAAA 2600:1802:4::1
;; Query time: 54 msec
;; SERVER: 208.80.126.2#53(208.80.126.2)
;; WHEN: Sun Mar 14 14:16:57 MSK 2021
;; MSG SIZE rcvd: 355
Even if these addresses NS*.postcrossing.com points to the same IP addresses as .dnsmadeeasy.com domains, that’s not good, especially because all the DNS servers for COM domains should be registered in the zone to obtain a glue records, but the NS.postcrossing.com are not registered.
It will be better if you replace NS records for the POSTCROSSING.COM in the DNS zone at *.dnsmadeeasy.com DNS servers to make NS records the same as the DNS servers addresses the domain is delegated to.
Sometimes (in very rare cases) such situation could cause the unpredictable situations (I have some of them in my more than 10 years work at the domain registrar company).