One of the most discussed problems at our support hotline is, where to install our ExSBR transport agent to implement sender based routing in an Exchange Server organization. This question is easy to answer for most customer situations, but the answers are sometimes hard to understand.
In General, we need ExSBR to be installed on the transport servers, which host external Internet SMTP Send Connectors. These are in general the Send Connectors with the SMTP:* Address Space. As Exchange Server will send all outgoing emails over these connectors, the emails will pass the servers, hosting the SMTP:* connectors. It is not predictable, which server will be used as a bridgehead. Therefore ExSBR must be present on all bridgehead servers of the SMTP:* connectors.
To minimize the required number of ExSBR licenses, you can always limit the number of bridgehead servers on your SMTP:* Send Connectors. To get high availability, we recommend to use at least two servers as bridgeheads in a multi-server Exchange organization.
If you have Exchange Servers with the Edge Transport role in place, then the same rules apply for the Edge Sync Subscriptions with SMTP:* address spaces. In this case, most customers opt for the installation of ExSBR on the Exchange Edge Servers instead of the Hub Transport Servers.