Vista - Running custom email dll on Vista

I was using cdosys.dll to send emails with rtf attachments and emails from vb app
This dll would not work on Vista. So i purchased a dll to send emails that works on Vista.
My installer requires Admin to install this dll.
I install to my vista Ultimate and everything works perfect.
A customer bought a new computer using vista ultimate always gets, errors and not always the same error.
We have tried just about everything and still no success.
What we have tried:
Different email addresses in the to and from.
Uninstalled my app from his machine and reinstalled with a new downloaded copy.
Disconnected his fire wall
disonnected his anti virus
Sending with no attachment.
Running as stand User and Administrator
I have even formatted my vista with a new vista copy to replicate a new clean machine, downloaded
a new copy of my app and reinstalled. Everything works on my vista as it should.
I have run out of things to check or try. What else could i try ?