Exchange Hybrid and external email

[Efff dd]

Hello

I enabled a ‎Private‎ group to "allow external senders to email this group"

I see it in the GAL. but its failing with a Remote Server returned '550 5.1.1 User Unknown'

What do i need to create on my on-premise exchange to get this to work? My on-premise i think talks to and smtp /spam filter server that probably needs to know about the smtp address.

[#2]

Define "private group"? Generally speaking, you'd want to toggle the -RequireSenderAuthenticationEnabled flag. But the error message above suggests a different issue.

[#3]

a new group under Outlook client https://support.microsoft.com/en-us/office/create-a-group-in-outlook-04d0c9cf-6864-423c-a380-4fa858f...
that is set to private and appears as a private group under the exchange admin center.

[#4]

Got that part, but not sure where on-premises Exchange comes into play in your scenario, as Groups are pure cloud objects? In any case, for a Group to start receiving external messages, you'd want to toggle the RequireSenderAuthenticationEnabled flag.

[#5]

Had to create New-MailContact on premise AD