Change Domain Relay Type in Office 365
Some companies have a split mail organisation on Office 365, that is some users on Office 365 and some on a different mail server but both with the same domain.
If you do not correctly configure your domains you will find that email to users on the non-Office 365 platform may bounce as Office 365 cannot find any users on that domain.
Luckily, we can change the domain type in Exchange Online to force Office 365 to forward the mail for the MX records of the domain (Usually your on-premises server).
Firstly, go to the Exchange Admin Center.
If you are using a Small Business Office 365 subscription, you can access the Exchange Admin Center using this workaround.
Then click Mail Flow on the left hand menu and then Accepted Domains at the top.
Double click the domain you want to change and change the Domain Type to Internal Relay Domain.