In Mail Preferences -> Filters -> Add a new filter.
When I add an action, only options are "Discard...", "Keep...", "Stop processing..." and "Forward...". THere is not an option for "File to folder" or similar.
SOGO configuration for Sieve is correct and Sieve server (Dovecot) is running (port 4190).
I have noticed that there is a file /etc/dovecot/sievec/dovecot.conf which has a single line (appart from comments):
mail_location = maildir:/etc/dovecot/sievec/Maildir
Point is, this maildir does NOT exist on my installation (Nethserver 7, upgraded to final from some RC1 or RC2, I don't remember).
There exists though a /etc/dovecot/sievc/Maildir maildir, so I changed the file /etc/dovecot/sievec/dovecot.conf to point to the existent maildir /etc/dovecot/sievc/Maildir, restarted Dovecot and Sogo, and voila!
Now I have some more options when creating the Filtering rules: "Send a reject message", "File the message in... (folder)" and "Flag the message with...(tag)".
It seems tha either the Maildir was created on the wrong path, or there was an error when writing the .conf file.
I have my problem solved, but I don't know what is the right solution, so I'd like to ask, and when I know I can submit a patch.