W2K3+IIS 6.0 SMTP: problems resolving MX records

Hi all,
I'm new here, I tried to search the forum to find any post that could help me. But I had no luck...

So I present you my problem:

I have a W2K3 Server with IIS 6.0 Smtp Server.
To deliver mail I use an external public DNS Server. When the server don't find an MX record (doing a type MX query) in the Preferred DNS Server it does a "type A" query on the same preferred dns Server. And I read this is normal.
The problem is that when the server switchs to the alternate(s) dns server it continues to do "type A" dns queries instead of "type MX", and so, even if it finds something, the A record address is useless to deliver mail...

Does anyone know if also this is normal? And if not how to force smtp server to do ONLY "type MX" queries?

Thank you,
hope that some angel could help me

Bye
ZZ

My only thought would be changing your servers to another DNS server(s) as I don't think you can force this. Of course the bad side of doing this is propogation.