NAT for SMTP relay server

Question

I would like to connect an 365 EOP to on-prem SMTP relay server using a NAT system. The on-prem exchange server 2016 doesn't have a public IP.
What could be the best way to do it?

Answer 1

Hi @Marc

From your description, I suppose you would like to route outbound emails through EOP.

Here is a link on this topic for your reference:
Set up connectors to route mail between Microsoft 365 or Office 365 and your own email servers

The general steps would be:

1. create a connector in Microsoft 365 to receive emails from Exchange Server
2. create a send connector on your Exchange server to route emails through smart host (Microsoft 365)

And since you are using NAT, please make sure port 25 is open in NAT rules to allow the mail flow traffic between Exchange server and EOP.

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