I have a public IP for sending e-mail for our corporate LAN. We have two firewalls behind the router.
We also have two mail servers. Exchange server relays outgoing e-mails to an Endian proxy that in turn relays e-mails to the internet. In coming e-mails are received by the Endian proxy before being send to the Exchange server.
On the LAN all outgoing traffic is NATed to the gateway. We have also allowed SMTP traffic to only go via the Endian from the Exchange IP, all other smtp from any IP iexcept the Echange server is diallowed to relay through the Endian. Problem is that zen.smaphaus keeps blacklisting our IP. We have checked all machines for virues and worms and are convinced they are clean.