we are using roundcube 16.11
we have configured this as smtp server $config['smtp_host'] = 'smtp.ourdomain.com:587';
which works perfect when sending and replying.
in managesieve you can have an out-of-office. that messages is not send via the configured smtp server, but via localhost
the localserver doesnt have a dkim server, the server configured via smtp_host does.
why is roundcube not using the correct smtp server?
This is controlled by the managesieve/imap server, not Roundcube.