Something did change indeed.
We, as a small volume sender are seeing 451 4.7.650 S775 errors for the
first time.
No blockage. Maybe they switched over from sending the 451's to any IP
in the /24
and actually sending it to the IP.
But just a thought.
Mit freundlichen Grüßen
Lukas Adrian Kron
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Am 24.02.2026 um 14:47 schrieb Dominique Rousseau via mailop:
Le Tue, Feb 24, 2026 at 12:45:09PM +0000, Gellner, Oliver via mailop
[[email protected]] a écrit:
Microsoft's spam handling is broken, simple as that. No need to
argue about that.
We receive the same error messages for all servers since 2026-02-23
18:45 UTC. It's unlikely that Microsoft suddenly received spam from
all IP addresses of all kind of unrelated organizations yesterday
evening. Probably some algorithm went nuts there.
Browse the list archives.
Those errors when sending to "outlook" adresses run for a while now.
Seems like they made new rules with dynamic throttle, which nobody
understands for now.
For the servers (low volume) I run which encountered the probleme I
limited to one SMTP connection every 10s.
( which would probably bothersome for quit high volume sender, OK for us
)
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