You cannot do that. However you can create an outgoing filter that deletes some/all messages after being sent.
When you want to delete all messages after you sent them you could also choose to set the folder properties of 'Sent Mail' as follows:
General -> Activate 'maximum number of stored messages is'  and set it to zero.
Additional -> Activate: On exit 'remove old messages' and 'compress the folder'
This will mean you can check the messages you sent while you're still running TB, but in the next session they will be gone.