I have a set of public folder replicas on my Exchange Server systems. If someone from outside the organization sends a message to the public folder's SMTP address, how does Exchange decide where to send the message?
The answer varies. The public folder hierarchy, which defines the list of existing public folders and their addresses, is visible in Active Directory (AD), but the list of replicas for each folder isn't. When the Exchange Routing Engine service wants to know where to send a message that's addressed to a user mailbox, a simple directory query produces the name of the server on which the user's mailbox is homed. However, a similar query to find a public folder won't work because any given server might or might not have a public folder database, and that database (if it exists) might or might not have a replica of the given folder.
To solve this problem, the Routing Engine has special-case code that handles message transport. First, the Routing Engine chooses a public-folder database for initial delivery. It makes this choice by ranking all known public-folder stores from most-preferred to least-preferred according to whether the store is 1) on the local server; 2) in the same Exchange Server 2003 or Exchange 2000 Server routing group or administration group as the local server; or 3) on an Exchange Server 5.5 server. When the message is delivered to the chosen public store, if that store doesn't have a replica of the desired folder, the public store can find out which store does have a copy by checking the public folder replica list, and the Routing Engine sends the message there instead.
