Seconded - It's also very common with domains that are configured in
M365 as "internal relay"; they'll forward unknown recipients to any
connectors configured to send to a "partner org" (which can be on-prem
or another cloud filter). Ultimately this ends up causing a mail loop if
the receiving mail server of the connector doesn't reject the unknown
recipient outright.
This is an issue with the recipient domain's M365 configuration, and
you'd need their IT team to look into why the mail is looping.
- Mark Alley
On 10/2/2025 12:13 PM, Laura Atkins via mailop wrote:
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
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--
The Delivery Expert
Laura Atkins
Word to the Wise
[email protected]
Delivery hints and commentary: http://www.wordtothewise.com/blog
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