Mails which I have deleted in my email client (e.g. Outlook or Thunderbird) are still visible in the web interface at www.swissmail.org. How do I delete them completely?
In the "3-dots menu" right next to your user name you will find the option "Pack" which will definitely remove mails marked as deleted:
In addition, you can go to "Preferences" → "Server Settings" → "Maintenance" to activate the option "Compact Inbox on logout" to automate this process.