Please add below lines in Amavisd config file (/etc/amavisd/amavisd.conf) , restart Amavisd service and try again:
# Selectively disable some of the header checks
#
# Duplicate or multiple occurrence of a header field
$allowed_header_tests{'multiple'} = 0;
# Missing some headers. e.g. 'Date:'
$allowed_header_tests{'missing'} = 0;
