Sure, it can be like an option in the admin's area.
I have used the same feature for 9 years in Teamware Office webmail and I absolutely had no problem with everyday webmail using
This can keep a lot of free space in the mail storage area, because as I know my users, they don't empty their Trash
They even don't know it's possible. They just delete the message and that's it and they don't know the message is still in Trash
But if there is something like a popup with question "
Do you want to empty Trash?", most of them just click [OK] and Trash is empty
Sometimes you want to keep your messages in Trash, so you click [Cancel] and just log out.
So it would be really nice feature