The .STM file, or streaming file, holds non-MAPI information while the .edb file, or the Exchange database (essentially a large JET (Access-type of technology) database holds the messages and the MAPI information. If someone sends e-mail or accesses e-mail from a non-MAPI client (POP3, IMAP4, Web), that e-mail is stored in the .stm file. Mail from the internet is also non-MAPI and comes in a format called MIME. This information is also stored in the STM file. It may be converted to MAPI when a MAPI user (Outlook) accesses it and it is also converted to MAPI when the Move Mailbox wizard is used. There also may be pointers in the .edb to information in the STM.
Here are a few links that help to demystify the STM file:
(about 1/3 of the way down in the document the STM is discussed)
Native Content Storage in Microsoft Exchange
(Good article…communicates the idea behind the STM very well)
What causes a large .STM file?
Several things can cause a large STM file. One of those could be that your server is open as a relay. Sembee, an Experts-Exchange expert, and a Microsoft MVP, has produced some concise information.
Another thing that you could have in your system is a loop. Leaving Exchange up (and getting an outage approved), unplug the network cable temporarily. Take a look at your queues and see what gets queued up. Look @ both the SMTP messages and the messages in your MTA. Another thing to do is to enable message tracking and see who is sending a large amount of messages.
Look at your mailboxes and see what the largest mailbox is.
If you have a very large STM and a small EDB, something is wrong. Examples of exceptions are if you have an inordinate amount of OWA users, run a business that receives a lot of Internet e-mail, have a lot of POP3 users, who leave messages on the server, etc.
Some anti-virus scanners have even been known to cause problems with Exchange, both a the file level as well as Exchange-aware AV apps.
Please see this Experts Exchange question, and answer, regarding file-level AV scanners and Exchange:
One thing that you can do is look at the mailbox sizes, add them all up, and see if they are over xx GB. If they are, then you are fine. Even though Exchange has a single instance store, the user