In Office365, we have a distribution group,
firstname.lastname@example.org, set up with the following members:
- email@example.com (@gmail.com, for example)
If an internal user (i.e. an @company.com email address) sends an email to
firstname.lastname@example.org, all members of the group receive the email.
However, if an external user sends an email to
email@example.com, only the internal users receive an email. The external users don't even receive an NDR, even though I have ReportToOriginatorEnabled set to True on the group.
This question shows how to fix issue of emails not being delivered in Exchange 2010: Distribution list with external email sent from outside, however, the solution in the linked question was already in place. There was nothing for me to change, so I'm still experiencing the same issue.
Message Trace in Office365 reports that the email to the external member of the group was successfully delivered to the external SMTP server, but I have tested using Gmail and several other services, and none of them receive the email, or an NDR.