At the risk of being obvious, click on the first item in your sent items
folder while holding down the shift key. Then scroll to the bottom and
click the last item. That will select them all - or you can use any range
you desire. Then just click the delete icon or press the delete button.
Once done, you might want to empty the deleted items folder (assuming it
doesn't tell you there isn't enough room there and they'll be permanently
deleted). If you're sure you want to get rid of them, hold down the shift
key while clicking the delete icon. This will delete the selected items
without going through the deleted items folder - and you won't be able to
recover them.
If you want to save some of them, then after selecting them all, hold down
the control key and select the ones you want to keep. They will be
"de-selected".
