http://kb.mozillazine.org/Undelete_a_message

Thanks for the link. Unfortunately, I have no Idea what it's talking about. I attempted to use the Recover deleted messages add-on but it didn't add the Recover deleted messages to the sent folder. I also tried to follow the instructions in the POP account (mbox files) paragraph but couldn't get to first base with that. I need something much simpler in the way of help.

Did you check the Trash folder where deleted files normally go?

Airmail said

Did you check the Trash folder where deleted files normally go?

Yes. Not there.