So, had this problem.
Two domains share a VPS with Cpanel. They are on seperate NS. The NS is not hosted on the same server as the Cpanel.
Both domains use O365 for Email. Lets call them domain A and B.
Domain A – User 1 Emails Domain B – Users 2. Gets the email back from O365 saying that it’s invalid and I don’t have permissions. But the server responding is the Cpanel server….. The MX is pointing to O365 and all is dandy. What the hell does it have to do with this?
Make sure that “external email domain” is set on the domain in the Cpanel settings (click on the Mail Auth button). And turn off DKIM and SPF if you use another mailprovider.
Solved all the problems. But in my world it should be:
Domain host->NS->O365. But it seems as when both domains reside on the same Cpanel instance, it skips the part with the NS server and checks on itself first… But still very wierd. Worked afterwards.