Empty your email trash folder
Deleting an email message is a simple task of just swiping it off the side of the screen, but this doesn't actually delete the email message. Instead, it just moves the message to the folder that you set as you default trash folder. If your trash is set to the official Trash folder of your mail program, this probably gets cleared out on a regular basis automatically. However, if you set your folder to be something different, the size of that folder can get rather large and the only way to clear it out would seemingly be to do it from a computer. But that is actually not the case.
If you open up the trash folder on your webOS phone or TouchPad (the folder should have a icon next to the name), you can then swipe-down from the top-left to bring down the Application dropdown menu and you should have an "Empty Trash" option. Tapping on that option will bring up a pop-up asking you to confirm that you want to permanently delete anything in that folder. Tap the "Empty Trash" button and every email that is synced to your device in that folder will be deleted. It is important to note that only those emails synced according to your email retention settings will be deleted. Any emails beyond that timeframe will remain in that folder on your email system.