RPC Endpoint Mapper
The RPC End Point Mapper always listens on port 135.
When an Exchange server starts, each of its services (not HTTP, SMTP, POP3, or IMAP) is dynamically assigned to a port. The port number of Exchange server listens on can change every time that the Exchange server starts. When an Exchange server starts, it registers its port number to RPC End Point Mapper.
When Outlook starts, Outlook contacts the Exchange server's RPC End Point Mapper (port 135) for exchange service registered port number, e.g. 4001. And then, Outlook communicates with Exchange Service on port 4001.
Both Microsoft Outlook and Microsoft Exchange use the remote procedure call (RPC) protocol to communicate.
