If a user accidentally deletes messages or a message folder, you can easily undelete this within a few days.
The amount of time that messages are stored in the VPOP3 'recycle bin' is set in Settings → Database → Message Store. The default is 72 hours (3 days)
To undelete messages, go to the Users list, double-click the appropriate user, and go to the Advanced tab.
In there, in the Message Recycle Bin Size section, you can choose a date/time (within 15 minutes bands), and it will display how many messages have been deleted since that time.
You can select whether the undeleted messages will be marked unread, and starred or flagged (or however your email client displays the IMAP4 'FLAGGED' attribute). This can make it easier to find the undeleted messages if necessary. If you leave those two checkboxes empty, the undeleted messages will have the original attributes at the time the messages were deleted.
Once you have chosen the appropriate time, you can press the Undelete button. This will undelete all messages/folders deleted (for the chosen user) since the specified date/time.