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Marc-8505 avatar image
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Marc-8505 asked emilyhua-msft edited

NAT for SMTP relay server

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?

windows-server-2016office-exchange-server-mailflowoffice-exchange-server-connectivity
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1 Answer

KaelYao-MSFT avatar image
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KaelYao-MSFT answered KaelYao-MSFT commented

Hi @Marc-8505

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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Hi @Marc-8505

I am writing here to confirm with you how thing going now?
If you have any questions or needed further help on this issue, please feel free to post back.

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