I ended up going down this rabbit hole a few years ago. I finally came to the conclusion that this was a O365 tenant that had a second on-prem (or even cloud based) spam filter / appliance that was rejecting the forward. And then it would be looped and come back to O365 and on and on and on until it got hop count exceeded and O365 would reject the message.
I think one of the reasons I came to that conclusion was I was seeing a lot of rejections from some other appliances at the same time. But I don’t remember all the details. Not much you can do about it. I recommend clients suppress the addresses at least temporarily until we got all the blocks on them cleared. laura > On 2 Oct 2025, at 17:41, Scott Q. via mailop <[email protected]> wrote: > > One of our customers is getting this message when e-mailing a specific > address hosted @ MS365: > > 52.101.41.26 failed after I sent the message. Remote host said: 554 5.4.14 > Hop count exceeded - possible mail loop ATTR1 > [SJ1PEPF00002313.namprd03.prod.outlook.com > <http://sj1pepf00002313.namprd03.prod.outlook.com/> 2025-10-01T14:52:23.190Z > 08DE008E05DFD276] > > but it works fine if we use a different address as source, aka a random Gmail. > > Anyone else saw this and know what it might be ? > > Thanks! > Scott > > _______________________________________________ > mailop mailing list > [email protected] <mailto:[email protected]> > https://list.mailop.org/listinfo/mailop -- The Delivery Expert Laura Atkins Word to the Wise [email protected] Delivery hints and commentary: http://www.wordtothewise.com/blog
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