sendusing in smtp why?

[chamling]

Hi,
I got Error "The "SendUsing" configuration value is invalid." while
using following code.
MailMessage sendinfo= new MailMessage();
sendinfo.BodyFormat=MailFormat.Html;
sendinfo.Body="bodyvalue";
sendinfo.To="emailadd@mydomain";
sendinfo.From="emailadd@mydomain";
sendinfo.Subject="Preview Newsletter";
SmtpMail.Send(sendinfo);

Therefore, I added a line.
SmtpMail.SmtpServer="localhost";
It then gave me error "The server rejected one or more recipient
addresses server response was: 550 5.7.1 Unable to relay for
myemailaddress"

I saw a few message threads saying that the email address has to be in
same domain to get this to run and it already is.

I have tested this code in 2 servers. It works at one even without
even the smtpserver specifiation and it doesn't work on the other
server no matter what I try. I have tried several methods
e.g.CDO.Message() and non seem to work on this server. It has to be
something with configuration. I dont understand. I have spent a lot of
time on it.
Any suggestion will be great.
Thanks
Dee