On Wed, Jun 17, 2026 at 05:52:34PM +0100, Paul Smith via mailop wrote:
> <[email protected]>: host mta7.am0.yahoodns.net[67.195.228.110]
> said: 550 5.7.25 Forward-confirmed reverse DNS failed tnmpmscs (in reply
> to
> MAIL FROM command)
Since the Yahoo server's address was IPv4, the client's IP address as
seen by Yahoo must also have been IPv4. From the perspective of DNSViz
the requisite records are present and consistent:
https://dnsviz.net/d/132.10.159.178.in-addr.arpa/ajTXXw/dnssec/
https://dnsviz.net/d/mx13.vpop3mail.com/ajTX3w/dnssec/
> No one else seems to be reporting any FCrDNS errors from that server, and
> the errors only just started today, with no configuration changes at our
> end.
Transient path-dependent or observer-dependent errors do happen from
time to time. There's not much you can do about that unless the
other end is willing to investigate. Open a ticket and pray.
> The 'tnmpmscs' in the error message looks odd - it doesn't mean
> anything to me, but it's in all the error responses
Gemini tells me:
"tnmpmscs" is a known string, but it is not a standard industry
acronym. Instead, it is an internal host identifier or signature tag
used specifically by Yahoo's mail infrastructure (which also handles
mail routing for AOL, Verizon, and related domains).
...
You will typically see it at the tail end of lines like these:
450 User is receiving mail too quickly tnmpmscs
452 Too many recipients tnmpmscs
...
--
Viktor. 🇺🇦 Слава Україні!
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