White-listing and blacklisting doesn't work in Horde webmail.
Horde blacklisting filter has a special way of applying blacklisting filters. Filters are not triggered upon receiving new email, but are manually applied.
Packages should be installed, namely:
psa-ingo psa-kronolith psa-mnemo psa-passwd psa-turba
It is needed to press the 'apply filters' in Horde interface under
Filters and email items from blacklisted senders would be removed/moved to spam.