There is a way of recovering deleted mails in inbox or sent items. first you need to check the deleted items policy in the exchange system manager. to get there>
open exchange system manager>Servers>mailbox
check the mailbox store properties>limits tab and then check the deletions settings.
choose how long u want to keep the deleted items. in that case you will be able to recover them before that time expires even if a user deletes them permanently,
That is true, however if they are not deleting the sent items, and just have the box to “save a copy of my sent items in my sent items folder” unchecked then recovering those sent items is not possible. I am afraid the only way you are going to achieve your goal in this case is through a third party tool, but of course those are not free.