Tracing down an issue of "why are we no longer receiving owner notices" such as list membership changes, moderation list items, etc. we are seeing errors in MS365 side of things about "failing to deliver".
Upon deeper trackdown of this, our owner user mailman@example.com is seeing messages sent to it from @lists.example.com and @distro.example.com from mailman@example.com (despite the From in the envelope being list-bounces+mailman=example.com@[lists|distro].example.com)
Is there any specific reason for this? Is it because the MS365 side is ignoring the Envelope's From address and relying on the actual *from* field which is breaking things because MS365 users are "not allowed to send as [lists|distro].example.com"? This seems highly suspicious if this is the case, and I want to see if there's anything we can do on the Mailman side of things to make this 'work' for the 365 recipient (mailman@example.com is a shared mailbox for the IT team in this case, so it's a legitimate user).