In Outlook 2003 SP3 I did the following
1. Went to Tools -> Advanced Find -> Advanced (Tab)
2. Selected Field -> All Mail Fields -> Message Class
3. Used "Message Class" -> "Contains" -> "EnterpriseVault" and hit Add To List
4. Hit the Browse so it looked at "Malbox - testUser"
5. Checked the "Mailbox - TestUser" box and then checked "Search Subfolders"
it found items in the sent items no problems
https://www.linkedin.com/in/alex-allen-turl-07370146