Mail Flow in Hybrid Deployment When HQ Goes Down

Question

Hello,

We currently have a Hybrid Exchange deployment with three Exchange Server 2019 Mailbox servers in our HQ, and we also have a DR site with another three Mailbox servers as part of the same DAG.

I already understand how mail flow works between our HQ and Exchange Online under normal conditions. However, I would like to clarify how mail flow will function in a Hybrid setup if our HQ goes down for any reason. Specifically:

1. How will mail be routed if HQ becomes unavailable?
2. Will the DR site be able to handle mail flow to and from Exchange Online automatically?
3. Are there any additional configurations or considerations to ensure seamless mail flow from DR in a failover scenario?

Any guidance or best practices would be greatly appreciated.

Answer 1

Where is the File Share Witness? If its in a 3rd location that remains accessible, then the DAG will stay online in the DR site.

If the FSW is in the main site and its down, and then the DAG loses quorum, you will need to do a datacenter switchover:

https://learn.microsoft.com/en-us/exchange/high-availability/manage-ha/datacenter-switchovers?view=exchserver-2019

Either way, its on you to ensure DNS resolution and routing are online to handle the new routing to the DR site if the main site goes down.

Personally, I dont like DR sites. Both sites should be considered "prod" and your architecture should be designed to handle the databases and activity in both sites as "active". The FSW should be in a 3rd location to allow for automatic failover and HA.