I do also see these warnings in our dockerized installation every couple of minutes.
Since everything else is working fine I had ignored these warnings so far.
Nevertheless I would also be interested why these warnings do occur.
in master.cf I do see:
# start whitelist_fwd
127.0.0.1:10027 inet n n n - 0 spawn user=nobody argv=/usr/local/bin/whitelist_forwardinghosts.sh
# end whitelist_fwd
and in main.cf I do see:
postscreen_access_list = permit_mynetworks,
cidr:/opt/postfix/conf/custom_postscreen_whitelist.cidr,
cidr:/opt/postfix/conf/postscreen_access.cidr,
tcp:127.0.0.1:10027
Both cidr files do exist and have valid entries.
