Roundcube via Mailu and Docker is not sending Mails

[Spielesocke]

Hi there, i have a mailu Server and want to use Roundcube with it. The Mailu server runs with a DNS resolver outside the Docker structure and can seems to run fine. But when I log in to Roundcube and want to send an email, I get an error message: SMTP ERROR, Sender cannot be set. So i search the logs of the smtp and find this: warning: connect to Milter service inet:antispam:11332: Connection refused