problem with email client

Hello

I have developed a mail sender program using cdosys.dll in visual c++. It takes SMTP address and send mail. It works fine with SMTP server provided by ISP.
The problem is that when I use MS Exchange server SMTP address to send mail with in LAN , SMTP througs an erro.
Exact error description is

" Message could not be sent to SMTP Server. The transport error code was 0x80040217. The server response was not available."

MS exchanges server works fine with my outlook cllien but it gives error when I use my own program.

Can anybody help me out what is problem. Is there any setting needed on exchange server side.
Thanks

imran