Simply put, Conditional Mail Routing, also known as Criteria Based Routing, is a way of configuring Exchange Online Protection [EOP] connectors in order to send or receive mail a certain way based on the condition of the individual email. For example, we can force TLS for a specific sender or route email based on recipients’ properties to different email server locations.
I was recently working on a global Exchange migration to Office 365. This particular client had an Office 365 tenant hosted in Europe with a Hybrid deployment in India. On top of that, there was a separate Exchange organization deployed across the United Kingdom and the United States of America.
For us to be able to update the MX records to point to the client’s Office 365 tenant, two requirements had to be met because of security and legal reasons:
- Requirement 1: emails sent from partners to certain business application mailboxes (hosted on-premises in the UK) had to go directly from Office 365 to the Exchange servers in the UK, i.e., without being routed through India;
- Requirement 2: emails addressed to US users had to go directly from Office 365 to the Exchange server on-premises in the US, i.e., without being routed through India.
To see how we can meet these 2 requirements using Conditional Mail Routing, please check my Exchange Online Protection Conditional Mail Routing article at MSExchange.org.