When you do not use a smart host a recieving mail server may do a reverse DNS check.
My understanding is that it uses the source IP of the email (In our case the primary IP of our ISA firewalls external interface) and checks the DNS PTR record to see if brings up a DNS MX record that matches the domain of the email. Now my question is when you send mail via a smart host either at your ISP or elsewhere like AuthSMTP what happens to reverse DNS checks. The way I see it is that the reverse DNS check will take the source IP address which will be the smart host address and not be able to associate it with an MX record of the emails domain. Can anyone let me know if I'm missing something.