Forum Discussion
Adding a new domain (company acquisition) to existing Office 365 Environment
When you add the domain, you need to make sure to configure it as Internal Relay in Exchange Online. Otherwise, for any address associated with that domain, Exchange Online will attempt internal delivery, and will fail if no matching account was found. When you set the domain as Internal relay, it will still check for internal matches, but if no such are found it will redirect the message to where the MX record points at currently.
Hi Vasil,
Thanks for your reply. I was reading this documentation for the type of connectors
could you please point me out to the right one?
So although no DNS record was changed, just the TXT record added to domain ownership approval, and no service was enabled for the new domain it was an expected behavior?
T
- Hi!
Vasil is right. Unless you add a send connector and set the domain to internal relay then if the existing users on 365 send mail to that domain it’s going to look internally within Office 365 and bounce.
So,
1./ Add domain, txt and validate and add the Exchange service, do not change any other DNS records
The domain goes onto Exchange Online at this point. If you don’t do steps 2 and 3 you may start experiencing bounce backs if users send mail from the existing users to the newly added domain. This is expected behaviour.
2./ Go into Exchange Admin Console and in Accepted Domains change the newly added domain to internal relay domain
3./ In The Exchange Admin Console Add a outbound connector (from Office 365 to the existing mail platform) for the newly added domain pointing to the highest priority MX record on that domain.
https://docs.microsoft.com/en-us/exchange/mail-flow-best-practices/use-connectors-to-configure-mail-flow/set-up-connectors-to-route-mail
That should work and ensure mail flows back to the new domain’s mail platform from Office 365 users.
You may want to do this change out of hours or notify the existing 365 users so they do not send mail between the domains at the time you make these changes.
Hope that helps
Best, ChrisHi Chris, I really appreciate your reply, in step 1 should I enable the Exchange services to proceed or I can leave it no configured.I am confused because we haven`t migrated any mailboxes from their end and haven`t decided if we going to do a cut-over or hybrid migration yet.
Thanks
Aysan
- Yes, you should enable Exchange services to proceed otherwise the domain will never go onto Exchange Online and you will not be able to do steps 2 and 3.
If you are unsure about the migration route to take, I would advise not to proceed with adding the domain until you have a clear migration methodology. As Vasil said, it is only 200 users so this usually fits a cutover migration. You can check out the ways to migrate to Office 365 here
https://docs.microsoft.com/en-us/exchange/mailbox-migration/office-365-migration-best-practices
https://docs.microsoft.com/en-us/exchange/mailbox-migration/mailbox-migration
And follow this guidance.
Best, Chris
