How to Shrink .STM File DB on MS Exchange Server 2003

Hi. We are using MS Exchange Server 2003 running on POP3 Clients. Our current DB size is 33GB (31GB .STM file and 2GB .EDB file).

Question: How do we reduce our database size especially the .STM file?

We did delete unnecessary emails on every users mailboxes, delete unnecessary users mailbox on the store, and do also offline defragmentation.

But after offline defragmentation, database size seems untouched and result to the same size as before.

Is there anything I missed to perform to fully reduce our exchange database size?

any help would be highly appreciated.

Many thanks,
- James