The Edit User → Advanced tab contains some 'advanced' settings for a VPOP3 user.

In VPOP3 Enterprise (v5 and later) the Message Recycle Bin will keep messages in the message store for a short time after the user has deleted the messages. This is to allow easier recovery from the inevitable case where a user has deleted messages by mistake, and wants them back.

To use this option, you choose a time from the drop-down boxes from just before the messages were deleted, optionally choose whether you want the undeleted messages to be marked in some way, and then press the Undelete button.

You cannot undelete only specified messages; you can only undelete messages deleted since a specified date/time

To change how long messages are kept in the 'recycle bin', go to Settings → Database → Message Store, and check the Message Recycle Bin section there.