2025-09-12 14:47:06 host hotmail-com.olc.protection.outlook.com [52

           .101.42.7] SMTP error from remote mail server after end of data: 55

           0 5.7.515 Access denied, sending domain INOVA.ORG doesn't meet the

           required authentication level. The sender's domain in the 5322.From

            address doesn't meet the authentication requirements defined for t

           he sender. To learn how to fix this see: <https://go.microsoft.com/f>](https://go.microsoft.com/f)

           wlink/p/?linkid=2319303 Spf= Pass , Dkim= Pass , DMARC= Fail [DS4PR

           01MB9298.prod.exchangelabs.com 2025-09-12T14:46:23.919Z 08DDF092DDB

           F8591] [BY3PR05CA0055.namprd05.prod.outlook.com 2025-09-12T14:46:23

           .939Z 08DDF1BF631DE849] [MWH0EPF000971E3.namprd02.prod.outlook.com

           2025-09-12T14:46:23.954Z 08DDF09733AC3A2A] --1757688385-eximdsn-159

           7447190

In this case some  some non profit board members are sharing and some of them have a Hotmail address.  When the guy at INOVA.ORG sends a message to the listserve, Hotmail overreacts and rejects the message.  So the Hotmail guy misses the message (not even in the spam folder).

Plus, if the INOVA.ORG gal is sending a lot of messages to the listserve, then that is going to bounce off all the Hotmail guys.

I guess that is one way to solve the problem, penalize your own subscribers, and just bounce them off.

I'm not seeing a DMARC policy for INOVA.ORG , so I'm not sure why Hotmail is claiming

doesn't meet the authentication requirements defined for the sender.

Hotmail should be looking up mail-list.com to see what it's DMARC policy is, not looking up INOVA.ORG.

As explained at

https://help.mail-list.com/m/59124/l/946163-dmarc-how-mail-list-com-handles-dmarc-reject-settings

Mail-List.com will automatically compensate for any domains that have set up DMARC policies of Reject or Quarantine. A detailed explanation is listed at the link above..

I see the messages that triggered Hotmail

xxxx-board 2025-09-12 14:45:20 [email protected]

MXtoolbox.com says

inova.org email hosted by mxa-009ec301.gslb.pphosted.com

Which I'm guessing is Proofpoint.

My guess is that inova.org is experimenting with DMARC and has it turned on somewhere and it leaked to Hotmail either by mistake or on purpose.

I'm happy to dig further and open to other's insights.