Transport Rules and Hub Transport servers

A good discussion came up today on a mailing list I am on. I thought some of the information from this was relevant and should be shared.

• Microsoft has tested up to 1,000 transport rules with the RTM release of Exchange Server 2007. You could scale past 1,000 transport rules with more RAM and faster processors in the Hub Transport servers. And, of course, more rules=more AD replication.
• If a message crosses more than one Hub Transport server, the rules "fire" only when the message hits the first Hub Transport. An internal header is added to the message that tells other Hub Transport servers that the transport rules have already been applied to that message. This header is stripped if the message is sent outside of the organization.
• If a message is bifurcated during the categorizatoin process (such as breaking a single message up in to to messages - one for internal recipients and one for external recipients), then the transport rules apply to each messages separately.