Action: Ensure mail flow bypasses your mail gateway For Exchange hybrid mail transport to work, the on-premises SMTP endpoint ...

Action: Ensure mail flow bypasses your mail gateway For Exchange hybrid mail transport to work, the on-premises SMTP endpoint that accepts connections from Exchange Online Protection (EOP) â€" the service that handles transport for the Exchange Online organization  â€" must be an Exchange 2013 Client Access server or an Exchange 2013 or Exchange 2010 SP3 Edge Transport server. There can be no other SMTP hosts or services between the on-premises Exchange 2013 Client Access servers or an Exchange 2013/2010 SP3 Edge Transport server and EOP. Learn more at Understanding Transport Options in Exchange 2010 Hybrid Deployments.