By default, in Horde Webmail any messages deleted will be marked but not moved to the Trash. This means that when an email is marked as deleted it continues taking up space. It will also remain in your inbox which makes it harder to find the emails you need. You can modify this setting so that deleted messages are placed in the Trash automatically.
First, log into your Horde Webmail. Once you have logged in, click on the Options button at the top of the page.
At the top right of the "Options" page, in the dropdown menu select Mail:
Next, under "Message Options" select Deleting and Moving Message:
Now, check the box next to "When deleting messages, move them to your Trash folder instead of marking them as deleted?":
Finally, hit Save Options. Now any mail that you delete from your inbox will automatically go to your Trash folder. You can then click the Empty Trash icon to permanently delete the emails: