I'm about 10% through a staged migration from Exchange 2003 on-prem to Office 365's Exchange Online.
The question I have is how do the migrated users that are now successfully using Exchange Online send email to the users still on the on-prem server?
When they try they receive an NDR with the error "Remote Server returned '550 5.1.1 RESOLVER.ADR.RecipNotFound; not found'".
I only have the users that have been migrated have an O365 user account. Those who have not and are still on-prem do not have a O365 user account – yet.
How do I configure the setup (in O365 I assume) so the email addresses for accounts that are still on-prem get delivered to our on-prem server rather than just failing delivery because the address doesn't exist in O365?
I did a little reading on similar scenarios and about the 550 5.1.1 error and have now changed our organisations domain in O365 from authoritative to internal relay. And I was required to create an outbound connecter (in O365).
Emails now sent from bob@domain.com (who is migrated and using O365) to barry@domain.com (who is still on-prem and DOES NOT have an O365) account are not coming back with an NDR, but are not being delivered either…
Maybe I have the outbound connecter misconfigured?
Please help if you have any suggestions on how to fix this.
Best Answer
You need to setup one of the exchange servers as an edge server, ie it needs to receive all email and then forward the email on. O365 is not aware of barry@domain.com mailbox, so you need to setup o365 to deliver unfound emails to your onprem box. Alternatively setup a mailbox for barry on o365 and use pop account from the on-prem to fetch email.