450 error when sending email through IIS, SMTP

Question

One of our servers is running a website that sends out email addresses with logins and passwords upon a user's registration. The email is configured with CDO. We seem to be getting blocked by some email services (see below). Is there a way to configure either rDNS, DNS, PTR, or SPF records to avoid getting bounce-backs?

Our company's public IP is x.x.x.195

The IIS server name is Server1a; private domain is private.com

The site it is sending from is public.com; public IP is x.x.x.140

We are getting bouncebacks from ATT, AOL, and other email services. Here is an example of one:

Message delivery to the host '204.127.217.21' failed while delivering to the remote domain 'att.net' for the following reason: An SMTP protocol error occurred. The SMTP verb which caused the error is 'MAIL'. The response from the remote server is '450 service permits 2 unverifyable sending IPs - server1a.private.com is not x.x.x.195

Answer 1

Problem was a greylisting due to higher requirements by Yahoo, AOL, and several other mail systems. The issue was fixed by updating the SPF record and creating a reverse-dns PTR record that would resolve to our server name.