managesieve plugin not working

[mcogre]

Hi there,

i installed Roundcube with the official Docker image roundcube/roundcubemail from Docker Hub and use it to access mail from an already existing mail server.
Unfortunately, this server does not support filter rules for mails. So, I installed the managesieve plugin to create filter rules in the Roundcube server.
Now, when I want to use the filter options, I get the error message "unable to connect to server". My research suggests that i have to install managesieve on the "mail server", but I can't find information on how to install and configure this for my instance.
Is there anyone that could help me?
Thanks a lot in advance!