How to install kasapi soap client for thunderbird autoconfiguration - grzchr15/kasapi-soap-client GitHub Wiki
Use https://www.kasserver.com to create a subdomain
Main domain example.de Autoconfiguration domain autoconfig.example.de
point to path /xx/pages from repo to root of website autoconfig.example.de
at /xx/pages/admin create a config_autoconfig.example.de.php with some updated values from config.php
Use Thunderbird to setup account, volia mailloginname and SSL servernames get automatic configured