Sharing Links fail when sent to Hybrid O365 Tenants or those using 3rd Party SPAM Filters
When a Shared Link (e.g. document, etc.) is sent from a user in one Office 365 tenant to a user in different Office 365 tenant, the expected behavior is to have the document open without needing to logon.
When sending a Shared Link to a user in an Office 365 tenant which is either in a hybrid configuration or if the tenant is using a 3rd party SPAM filter (e.g. Barracuda Cloud), use of the link fails with an error of: "User not found in directory".
We've looked at the MX records of these users and in each case, the destination tenant does not list Exchange Online as the primary destination for messages (this is expected).
I'm guessing the problem may center around the fact that during the process of opening the Sharing Link, the process invokes a DNS lookup against the intended user's email address (e.g. email@example.com). Since the resulting DNS lookup doesn't explicitly point back to Exchange Online in these cases, the remainder of the process fails to ultimately reach Azure Active Directory for the user lookup, hence resulting in the error message of "User not found in directory".
If this is indeed the case, what can be done to help Office 365 users in this situation?