in hestiaCP Configure Server: SPAMASSASSIN I edited the spamassassin config file /etc/spamassassin/local.cf
# the opposite; blacklisted mails can also save CPU
#
shortcircuit USER_IN_BLACKLIST on (I removed the '#')
# shortcircuit USER_IN_BLACKLIST_TO on
# shortcircuit SUBJECT_IN_BLACKLIST on
blacklist_from *@birdieapps.com (I added this line)
but there is a pre-existing empty
/etc/exim4/spam-blocks.conf and
/etc/exim4/white-blocks.conf
about which I find very little info (so I guess 2 more empty files in that dir are ok), as if this is an underdeveloped feature