I'm seeing rejection messages from the Microsoft galaxy of email providers.  For example

550 5.7.515 Access denied, sending domain ATT.NET doesn't meet the required authentication level. The sender's domain in the 5322.From address doesn't meet the authentication requirements defined for the sender. To learn how to fix this see: https://go.microsoft.com/fwlink/p/?linkid=2319303

In this case some car enthusiasts are sharing car tips and some of them have a Hotmail address.  When the guy at ATT.NET 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 ATT.NET guy 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 ATT.NET , so I'm not sure why Hotmail is claiming

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

In the example, the From: header looked like

From: Sam Jones (SJ66 at ATT.NET) <[email protected]>

The part inside the angle brackets <>, ([email protected] in this example) is the controlling address.  All other text is considered Display Only.

I'm looking at Addr-spec specification in RFC2822.

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