A preview feature would be very useful. Something to allow the user to see one or all of the messages sent before they are mailed would prevent user-error by allowing the sender to view exactly how they will look once the custom fields and other merged fields are dropped in. A pretty email is nice, but not useful when it has the wrong name in the salutation. My situation today was not even user-error. Fields from recently used lists were being pulled even though it's not what I had specified. I would have seen the error and have been able to go back and see what was going on before I mailed to a group of regular donors.