@esackbauer Thank you for the hint.
It seems not very smart for me to install Thunderbird only for managing sieve rules for my account on the mailcow server. The smart place would be the mailcow UI.
This is how it looks like in roundcube in Kolab (the corresponding UI to the mailcow UI and SOgo):
The generated sieve rule is /var/lib/imap/sieve/domain/w/.../l/.../Kolab.script
require ["fileinto"];
# EDITOR Roundcube (Managesieve)
# EDITOR_VERSION 9.3
# rule:[lists.kolab.org]
if anyof (header :contains "to" "lists.kolab.org", header :contains "cc" "lists.kolab.org")
{
fileinto "news/kolab";
}
I am currently migrating from Kolab to mailcow and we are really missing this possibility.