A computer running Microsoft Exchange Server that acts as an endpoint of a connector joining two Exchange sites. A bridgehead server is responsible for routing messages through that connector. There are two different types of bridgehead servers in an Exchange organization, although a single server often functions in both roles:
Pass-through traffic consists of messages that originate in one site and are destined for another site, but in order to reach their destination site, must pass through one or more additional sites.
You can reduce the effects of pass-through traffic by using dedicated messaging bridgehead servers—that is, bridgehead servers that have no mailboxes or public folders homed on them. You can also have traffic enter a site by one server and leave it by another.
If you have a large number of sites, you might want to configure a given site to use a different bridgehead server to connect to each remote site for load balancing of messaging traffic. Also, you can schedule directory replication traffic to occur at slow periods during each day.