How does Exchange Server use to send public folder replication messages?

In a pure Exchange Server 5.5 site, Exchange uses only remote procedure calls (RPCs) for replication messages. In a mixed site (i.e., one that runs any combination of Exchange Server 2003, Exchange 2000 Server, and Exchange 5.5), Exchange uses RPCs for replication messages that pass to or from an Exchange 5.5 Message Transfer Agent (MTA) and uses SMTP (managed by the routing engine) for messages that pass between Exchange 2003 and Exchange 2000 servers. In a native-mode Exchange 2003 or Exchange 2000 organization, SMTP carries all public folder replication messages.