Background
When Exchange 14 was implemented in the fall of 2010 at Tufts, as an implementation team we agreed to forgo the auto-removal of messages placed into the trash as a way to make the transition from then Trumpeter-Mail to Exchange easier for faculty, staff, and students.
How it works
This is a standard 30-day retention setting that can be applied on a per mailbox basis. Once configured, exchange automatically removes a message placed in the trash bin from a mailbox 30 days after it is moved to the trash bin. This setting does not care when a message was sent or received, it only matters when a message is moved to the deleted items folder. This retention policy only takes place in the deleted items system folder (trash bin) in the mailbox.
Testing
The best way to understand this setting is to have your individual account, or test account configured to behave this way.