@timotheus UPDATE: I noticed that the LetsEncrypt TLS/SSL files were symbolic links (with a relative file path). This caused some problems, but now IMAP is partially working. Some ports are still not working:
-143 + STARTTLS -> Not working
-993 + SSL/TLS -> Working
-465 + SSL/TLS -> Not Working
-587 + STARTTLS -> Working
How can I fix Port 143 and 465? What am I missing? And still I am unsure why ActiveSync my the preinstalled E-Mail Application on Android is not working.