Aha! That makes a lot more sense. It's definitely a dumb way to do it.
I guess maybe they assumed everyone sending mail has at least a /24 all to themselves. In our case our most-affected customers are in fact only on small portions of /24s that are shared with other users we don't have control over [facility-provided IPs]. I do understand that "ip neighbors" can cause issues in some cases but generally that's a blacklisting that is visible and can be disputed/resolved. I wonder what the issue is this system was supposed to solve. I could speculate but it's not helpful. And just in case anyone missed it - I am sorry for coming across like a d*ck. I shouldn't have let my frustration get the better of me. Thank you, Michael Denney MDDHosting LLC http://www.mddhosting.com/ > On Feb 20, 2026, at 8:23 PM, Philip Paeps <[email protected]> wrote: > > On 2026-02-21 07:49:10 (+0800), Michael Denney via mailop wrote: >> I checked mail logs going back to December 2025 - and I see exactly zero 421 >> errors. As a matter of fact we have exactly zero 4XX responses from Outlook >> - network wide, but I do see 550's with the error 3150. >> >> Are you sure this is properly implemented? >> >> Feels half-baked as though it was AI vibe coded. > > I suspect it's implemented on a network boundary that isn't a sender > boundary. One sender in a /24 is getting the 4xx tempfails and an unrelated > sender in the same /24 gets a 5xx on first interaction. > > That's clearly stupid but it's obvious that spam filters are no longer > maintained by sentient humans. > > The good (?) news is that there are fewer and fewer humans who care about > email who host theirs at Microsoft. > > Philip
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