How to enable third party email relay in M365?

Rocky Heckman 25 Reputation points

My company uses a third party payroll processing company. We need to configure an email address that forwards questions about payroll to them, and we need their system to be able to send responses to those emails from their email domain through ours so that it looks like the response came from our domain.

The email forwarding from us to them is fine, that is just a forwarder, but how do we get M365/Exchange to relay their email through our domain to the requestor or even new emails from the third party to our users?
I've tried using a Connector, but that doesn't work, the inbound email from the third party just vanishes with no record of it.

How should I be doing this?

Microsoft Exchange Online Management
Microsoft Exchange Online Management
Microsoft Exchange Online: A Microsoft email and calendaring hosted service.Management: The act or process of organizing, handling, directing or controlling something.
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Accepted answer
  1. Kael Yao-MSFT 31,586 Reputation points Microsoft Vendor

    Hi @Rocky Heckman

    Did you follow this documentation to setup SMTP relay in Microsoft 365:

    How to set up a multifunction device or application to send email using Microsoft 365 or Office 365

    If you would like to setup a custom receive connector for the third party company, please refer to Option 3 part.

    If you followed the guide but it still didn't work, please post screenshots of the receive connector settings.

    (Don't forget to hide your personal information)

    And if the third party got any error message like unable to relay, please also post the full error message.

    If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment". 

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    1 person found this answer helpful.

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