Daniel Faber wrote:

As practical advice, I recommend against doing so. By doing so, you're solving the wrong problem. If your mail filter is accepting mail without validating the recipients, and passing it on to courier, you can bet that you're going to generate a *ton* of backscatter. Now, first, the problem is going to be that you're going to bounce a whole lot of mail to people who didn't sent it, but whose address was hijacked by spammers. Eventually, the problem is going to hit you, too. The mail queue on the firewall is going to fill with tens of thousands of messages that can't be delivered, because the recipient address is invalid, and can't be bounced, because the source address is invalid, too. Once that happens, valid deliveries are going to start taking a very long time, again.

Really, you want your "firewall" to validate recipient addresses.