How are .edb and .stm files related, and when is content moved from one to the other?
A message database's .stm file stores contiguous sequences of streamed content. MIME-format messages from the Internet have a unique property: They always arrive as a stream of bytes. In Exchange Server 5.5, the Internet Mail Service (IMS) accepts inbound MIME messages and writes them to a disk queue, in which Exchange converts them to the internal Transport Neutral Encapsulation Format (TNEF) that the Information Store (IS) uses. Content can be converted back and forth several times, depending on whether an Internet or Messaging API (MAPI) client asks Exchange 5.5 for the message. Each conversion, however, results in a set of disk I/O operations, so unnecessary conversions increase the I/O load on your servers, reducing performance.
Exchange 2000 Server fixes this problem by making the .stm file the default location for inbound Internet-protocol traffic. Exchange 2000 converts a message's header properties to Rich Text Format (RTF) and stores them in the .edb file so that MAPI clients can use them. This process is called property promotion. If a MAPI client later requests another message property (including the message body), the IS converts that property to RTF and places it in the .edb file. After conversion, the message remains in the .edb file, never to return to the .stm file.