[IMP 3.x/4.x] You can undelete messages any time up until you click on the Empty Trash or Purge Deleted buttons, or until any automated emptying of the trash occurs if you have set up such an option. To undelete messages (marked as deleted) from the inbox listing, check the checkbox to the left of the messages you wish to keep, then press the Undelete link at the top or bottom left of the page. When you are viewing a message marked for deletion, there will be a link at the start and end of the message to undelete that message.
If you are using a Trash folder, you can undelete the messages by moving them out of the Trash folder into another folder.