Assuming you were still in the old email format, how did you have your Deletion schedule set for the Sent folder? If you still have the old format, on the Inbox page go to Preferences>>>>Email>>>>>>Email Deletion Schedule. If the messages were deleted by that setting, you won't be able to recover them. If you are in the new format, how far back does your Sent folder go? Because in the new format there is no deletion schedule for the Sent folder. The messages should be in there until you delete them yourself.