Hi Wakko!
> Unfortunately, people behind firewalls can't use $interface_address.
In that case, maybe you can check for the sending host's address and
reject HELOs that give an IP address which doesn't match:
deny condition = ${if match{$sender_helo_name}{\N^\d+(\.\d+){3}$\N}\
{yes}{no}}
condition = ${if eq{$sender_helo_name}{$sender_host_address}\
{no}{yes}}
message = Invalid recipient
log_message = open proxy (HELO)
Andy