registration email broke with Postini

We have an installation of WordpressMU version 2.7.1 running on MacOS Server 10.4.11
We are a school and it is an internal webserver, we manage it. and it lives here on campus.

When we first installed it, the email functions worked fine - users were able to create accounts and blogs and their passwords were sent to them along with any other notifications (to moderate comments of anything like that).

In September we changed our spam blocking. We had been using an appliance called Ironport. We switched to using Postini. The initial email notifications with new passwords stopped going through.
Now, once a user is created, they get notifications but the initial notification of account info does not arrive. The user gets an error message to check the email address.

We have theorized that this is because our mail is now routed through a google server before it gets to our internal mailserver. I beleive there is a section in the php code that checks the mailserver DNS entry against the IP. Since they don't match up - our mail is going to a google IP first and rerouted from there, WordPress is giving us an error.

We have registrations locked down to our domain (we are a school). If we open to any domain, the user does not get the error to check the email address - but the mail never arrives to the outside address. If we lock down to the domain, the user gets the error.

We have been through the Postini settings and nothing is getting held up there.

My question is: Has anyone else experienced this probelm using Postini as a domain Spam blocker and if so, were you able to correct it, and if so, how?

If not, can somewhere point me to the file that checks the email address, I wondered if I could comment out the check mail function.