Office 365 email invites to On Prem only Exchange rejected.

[craigw]

Here is the setup:

Office 365 setup but not Exchange Online.

At present all the Exchange infrastructure is on prem - mx records etc all point to local.

Everything works fine except..

 

When invitations are sent from services such as sharing a Onenote Class Notebook - the following is happening.

Sign is as [email protected] - invite user [email protected]

Greg gets an invite from Joe - but as Greg and Joe are on different domains - invite actually comes from an MS email address on behalf of Joe and works fine.

When Joe invites another internal user - [email protected] - Office 365 sends the email as [email protected] because it sees Alice and Joe on the same domain (and I assume) assumes that Exchange is either online or hybrid at a minimum.

 

On prem exchange see the email FROM [email protected] and says "no way did that originate from me" and rejects as spoofed message - as it should!

 

So - 1 solution is to add the Exchange Office 365 IP list of 40 or so IP's and subnets to the Allowed IP list on the Exchange on prem boxes. This should work - but will obviously whitelist the WHOLE office 365 system to mail relay through the on prem. Not ideal.

 

Anyone else got this setup and found a way to get these invites working.

Wally