Did you follow this documentation to setup SMTP relay in Microsoft 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.
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