Hybrid mail Flow with third party email server

Question

Hello!

I have an on-premise Linux server and M365 tenant. I will migrate users from on-premise to Exchange Online. The MX is still pointing to on-premise email system. If the recipient is migrated to Exchange Online, the on-premise relay the email to the cloud. So If something send an e-mail to Exchange Online user through the on-premise e-mail server, the sender mail server will be the on-premise e-mail server. When the Exchange Online Protection receive this e-mail, it will check it the sender domain SPF, in this case the gmail.com SPF. The Exchange Online Protection will see, that the e-mail originating IP is not the gmail public IP, it will see that the originating IP the on-premise e-mail server public IP.

Will it detect the Exchange Online Protection these email as spam? How could it work correctly?

Thanks.

Answer 1

Use Enhanced Connector Filtering and set the Linux Server as the IP to skip

https://learn.microsoft.com/en-us/exchange/mail-flow-best-practices/use-connectors-to-configure-mail-flow/enhanced-filtering-for-connectors