Our organization is on Office 365 for email for quite few years and configured with a domain say '@abc.com'. Last year the company got acquired by '@xyz.com' which runs only on premise of Exchange. The leadership in our organization decided to come up with a new domain which would reflect both the companies '@abcxyz.com' so that our existing customers still see us in their email domain. As the email admins in the parent org(xyz) configured '@abcxyz.com' on their DNS so that all the "incoming emails" flows on-premise and created user accounts for each of our users and a mail forwarding on the individual mailbox towards '@abc.com' On Office 365, we have added additional alias for each user @abcxyz.com so that their primary SMTP reflects the new brand change and all outgoing mails carried the new domain when users sent emails.
All these were working good until the parent organization decided to migrate and move few users on-premise.
How we did - Remove the existing mail forward rule from parent org. Configure Mail redirection on individual's mail box in O365 to '@xyz.com' which is found under user's Mail Options.
After migrating, Internal emails from '@abcxyz.com' users gets received without issues.
Issue: when mail sent from a user at '@xyz.com' to '@abc.com'.
The email in this case supposed to go from on premise to Office 365 and redirects back to On premise (from redirection rule) but didn't. emails weren't received in user's mailbox No NDRs returned Message tracing on Office 365 shows emails redirected to destination When the redirection rule is changed to a different forwarding address or to a different domain, I could see mails getting delivered.