In webmail, there are settings you can adjust. For the most part, you should use these at your own risk. There are a few things you can and should do, though. To access settings, click the gear icon.
Empty trash and compact inbox when you exit webmail.
Click on Server Settings
In the Maintenance section, turn on both Clear Trash on logout and Compact Inbox on logout, then click Save.
To return to your inbox, click the Mail icon.