reject_invalid_hostname is old value and is duplicated
ISPConfig setup postfix with:
smtpd_helo_restrictions = permit_sasl_authenticated, permit_mynetworks, check_helo_access regexp:/etc/postfix/helo_access, reject_invalid_hostname, reject_non_fqdn_hostname, reject_invalid_helo_hostname, reject_unknown_helo_hostname, check_helo_access regexp:/etc/postfix/blacklist_helo
It has reject_invalid_hostname and reject_invalid_helo_hostname, but reject_invalid_hostname is old value for reject_invalid_helo_hostname (postfix < 2.3), so it could be removed.