We are currently migrating from Exchange 2016 to 2019 in a hybrid environment. We have 2 2016 servers both in our main datacenter, and 2 2019 servers, one in our main datacenter and one in our offsite datacenter. Backup datacenter has its own DCs that are replicas of our main datacenter's DCs. Exchange 2019 has been installed and updated to CU 11.
When I run the hybrid configuration wizard and select all 4 servers to be included in the send and receive connectors, everything completes and no errors appear. However, mail gets stuck in the DnsConnectorDelivery queue on the server in our backup datacenter. The NextHopDomain for the stuck mail is our M365 domain, domain.mail.onmicrosoft.com
As soon as I remove the server in the backup site from the send and receive connectors, mail flows correctly again.
I've done a lot of internet searching and it seems the issue has something to do with our MX record, but both domains have the correct record in their DNS.
What could be causing the issue? Any help is appreciated!