Outlook :: Data Store
From MS article Q232323
In Microsoft Exchange 2000 Server, a database (also referred to as a "store") is made up of two files, an .edb file and an .stm file. For example:
Mailbox Store = Priv1.edb Priv1.stm
The .stm file houses Internet content message streams as defined in Request for Comments (RFC 822), and the .edb file contains messages that are in MAPI format (Rich Text Format).
When an Internet mail message comes into the Exchange 2000 information store, the body of the message is saved in the .stm file, and the header information (From, To, Cc, Time Sent, and so on) is converted to Rich Text Format (RTF), and then stored in the .edb file.
MAPI-based e-mail clients (such as Microsoft Outlook 98 and Microsoft Outlook 2000) only see the header in MAPI format, and other properties of the messages are converted to MAPI as needed. Any request for a MAPI property not already available causes the information store to call the IMail content conversion engine to furnish it.
For example, if a MAPI client reads an Internet mail message (and does not make any changes to the message), conversion is performed only on the properties needed by the MAPI client.
If a MAPI client edits an Internet mail message, a full conversion (from Internet message stream format to MAPI) is performed on that particular mail message. In other words, the entire message (both header and body) are converted into a MAPI mail message. At that point, the entire message is converted to MAPI format and stored in the .edb file.
This process also works in the opposite direction; messages originating from MAPI clients have their outbound message properties converted. In other words, if a MAPI client sends a message to a recipient on the Internet, it causes a full conversion from MAPI to Internet message stream format.