Fixed with modifications in /opt/zimbra/amavisd/sbin/amavisd:

# $unix_socketname = '/var/amavis/amavisd.sock'; # old amavis client protocol
# $inet_socket_port = 10024; # accept SMTP on this TCP port
$inet_socket_port = [10024,10025,10027]; # ...possibly on more than one
$inet_socket_bind = '127.0.0.1'; # limit socket bind to loopback interface
...
@inet_acl = qw( 127.0.0.1 88.190.221.42 ::1 ); # allow SMTP access only from localhost
88.190.221.42 is my Zimbra server IP

Sam.