1.Log into the Horde Mail system.
2.Click the "Folders" icon at the top of the window.
3.Check the box next to "Inbox" or the mailbox that contains the messages you want to delete.
4.From the "Choose Action:" drop-down menu, select "Empty Folder(s)."
5.You will be prompted to confirm the action, as it is not undoable.
6.Click "Empty Selected Folders" to clean out the mailbox.
Depending on how many messages are stored in your Inbox, this operation may take a few seconds to several minutes to complete. The only catch is that if the mailbox contains too many messages, the operation will time out and will not empty the mailbox. This is what happened to me with my 500,000+ messages.