Well, I'm not sure if this works when you wanna send email from a yahoo account to a gmail one (or vice versa). you know, actually you need a valid smtp host (a smtp host on localhost will be good for sending and recieving emails on current computer, I dont think it will be usefull when you want send emails to another accounts.) like gmail.
do this :
<br />
SmtpClient c = new SmtpClient("smtp.gmail.com", 465);<br />
or
<br />
SmtpClient c = new SmtpClient("smtp.gmail.com", 587);<br />
and of course you'll need to authenticate. see this :
http://mail.google.com/support/bin/answer.py?answer=13287&query=pop&topic=0&type=f&ctx=en:search[
^]
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Regards
H.Maadani