Distribution Group with External Members

[BrianChernish]

We are seeing what I believe is strange behavior from a distribution group in 365 (this is NOT an Office 365 group).

I created a distribution group with 2 internal members and 1 external member.

I send an email to the group and all members receive it.

If the external member does a "Reply All" (because he wants all group members to see the response) my 2 internal members get it just fine, bet the external member does not get it.  It is being "bounced" by his Mail appliance because the email it sees has his email address as the sender, NOT the group email address.

Anyone have any ideas?

Brian

[#2]

That's something that needs to be looked at their mail appliance layer, most likely something like ARC?

[#3]

To follow up on @Vasil Michev it is an issue with the mail appliance. One of the scenarios I always used to come across was that the mail appliance was set not to accept mail inbound from org domain as this is typical of spoofing. This may be the case here.

Best, Chris