Hybrid Centralized Transport sending emails to EXO

Question

I've discovered that Microsoft hosted tenants emails we receive is going directly to our EXO tenant and using the Hybrid Outbound connector going directly to on-prem Exchange by passing our on-prem Email Gateway fitlers.   Also saw some quarantined emails in the EXO from EXO Antimalware scanning.  With Hybrid Centralized Transport, I need all mail flow to work just like before the Hybrid configuration based on our MX records.  None Microsoft tenants email goes directly to our on-prem Email Gateway for processing before delivered to Exchange on-prem.

How do I change this behavior for Microsoft hosted tenants?  I do not want email processed by EXO at all.   I need all emails go through our on-prem Email Gateway for processing published with our MX records.   This is currently bypassing our on-prem anti-virus, anti-spam among other safety features and using EXO's feature.

Answer 1

Hi,@JohnNaguib

Thanks for posting your question in the Microsoft Q&A forum.

You want all emails to be processed through a local email gateway for distribution with MX records.

When using centralized mail delivery, mail is not routed through the internal deployment organization; this behavior is by design. To forward mail, an exact copy of the original message is created and sent to an external recipient. The mail routing logic sees that this new mail originated in the local environment and does not send the mail back to the local environment. Instead, it routes it directly to the external recipient domain through Exchange Online Protection.

The official documentation has explained it:https://learn.microsoft.com/en-us/exchange/troubleshoot/email-delivery/hybrid-messages-not-routed-through-on-premises?source=recommendations

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