Office 365 domain using external email server

New Contributor



Our tenancy has a number of different domains associated with it, all subscribed to O365 Business Professional. For one specific domain (lets call it, email is still hosted by a third party, so when setting this up we turned off the EOL component licence for the domain.


Unfortunately when other domains within the same tenancy attempt to send to email address at, office attempts to deliver these locally within the tenancy even though email services are provided externally. Not overly surprising really.


Are we able to force O365 to do an external MX lookup for this domain? Or do we need to set up a connector? We've looked into the latter, but are a little confused at how to set this up.



2 Replies
best response confirmed by SimonNZ (New Contributor)

A connector is not ultimately required. What is required is that the domain is set to non-authoritative (Internal Relay) in the Exchange Admin Centre. I used to do this a lot of Coex migrations

Once set to internal relay which you can change in accepted domains or by PowerShell and assuming the MX is directed at the external mail platform it ought to resolve

However, if this doesn’t work by itself you add an external send connector from Office 365 to the mail server

Hope that helps and answers your question!

Best, Chris

@Christopher HoardThanks very much for the information. Worked perfectly, as long as I was patient waiting for the update to take effect.