Exchange365 - distribution list, SMTP relay

Question

I needed to setup a distribution group which receives and forwards a voicemail from our phone network and distributes it to those on the list. It took some time to figure out, but the voice message does reliably get to the distribution list and is received by those on that list.

The phone system uses the same email (eg. ******@message.com ) to package the voice mail and forward it to the Exchange365 distribution list.
The distribution list DOES distribute to anyone placed on the list and also allows those on the list to respond with the condition that "Senders inside and outside of my organization" is the option selected in Delivery Management.

If I choose to limit access to "Only senders inside my organization" The the email containing voicemail never arrives. I receive this message : "******@message.com on 3/4/2022 10:21 AM This message could not be sent. You do not have the permission to send the message on behalf of the specified user."

Placing ******@message.com in the distribution groups delegation to "Send on Behalf" didn't fix the issue.

While this works as we need it having some degree of security other than not knowing about it would be helpful.

Can anyone offer advice on how to adjust whats presently working to limit it to only those awarded access?

Thank in advance.

Answer 1

Allow "Senders inside and outside of my organization"

then create a transport rule that drops everything from external senders sent to that list except ******@message.com