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?