Imap over port 993 failed

I have just installed the Android version of DeltaChat and tried to connect our mailserver (Mailsite, Hochschule). Our mailserver is configured to use port 993 only (TLS). I can setup this port in Deltachat for imap but after some time it complains that it could not connect over port 143 (regular imap, that is blocked by our firewall)

DeltaChat should only use the configured port