MX lookup returned: lookup mpeks.tomsk.su on 173.230.155.5:53: server misbehaving
The :53 means that the error was during a NS lookup. The domain mpeks.tomsk.su currently has no NS records:
~ $ dig mpeks.tomsk.su ns
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 7063
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 1
The "ANSWER: 0" means zero NS records. It's not normal, it confuses groups.io software. Consider email addresses @ mpeks.tomsk.su currently invalid, misconfigured. A NS check tool finds multiple errors:
Coincidentally, the mail admin with an address @ mpeks.tomsk.su yesterday posted to the Russian-language mailing list for users of mail server software "Exim" I participate in:
I'll notify him.
Lena