>You want to share one dedicated external source IP address among >multiple Postfix SMTP clients. If there were only one dedicated >external source IP address, then a NAT router would suffice.
That would be my first suggestion. For a cheap experiment, get something like a Cisco E2500, configure it on the external IP, turn off the wifi, plug up to four mail servers into the LAN ports and see how it does. Any NAT box has to manage port numbers per connection, I don't know how many simultaneous connections it can manage, and they don't say what the limit is, so it may run out of connections before your mail servers do. On the other hand it's under $100, and if it works, you're all set. Before buying complicated proxies or gateways, I would revisit your assumption that you need more than one mail server. It shouldn't be hard to configure most mail servers to saturate an outbound connection, and if yours won't do that, the problem may well be a configuration problem, or something that you could solve with an SSD disk cache rather than an address sharing kludge.
