Steps to follow for purge delete.
- Horde > Preferences > Mail > Deleting and Moving Messages.
 - Tick the second checkbox “Move deleted messages to your trash mailbox instead of marking them as deleted in the current mailbox?”
 - Save the changes.
 - Select trash mailbox as deleted items.
 - Tick the checkbox “Display the empty trash link in the menu bar?”
 - Save the changes.
 
For your assistance you can visit the following link.
https://www.youtube.com/watch?v=UmpiODpyyFk