Came across this over on FailDesk and my how it brought me back to an incident I dealt with / created once. We had issues with the Exchange server filling up and while users weren't bad about deleting large emails or backing them up to local PST's, they never emptied their Deleted Items. This led to full mailboxes and serious performance issues.
I had the bright idea to instigate a group policy that emptied Deleted Items when they closed Outlook. This worked great and quickly managed everyone's accounts for them. Until... "All my important emails are gone!" What? Turns out. A user who, yes, had what I would consider important emails, turned out to use a rather peculiar method of record keeping. They stored emails they wanted out of sight that they'd dealt with, but might have to reference later, in the Deleted Items. What? When they were done with them for sure, that's when they'd manually remove them permanently. (These were emails I probably would have stored, not even deleted.)
So, that caused a bit of an incident. The user could not be convinced that storing emails in the trash was unwise. I believe I was able to restore the emails in question, but wow. Moving forward, I left the GPO in place and it solved a lot of email woes.