emails are moved to queue folder not to receiptent

My problem is SMTP Server (localhost) sends the email but it sits in the Queue folder.

So I see all my emails are in C:\Inetpub\mailroot\Queue

Any Ideas how can I resolve this ,i developed application using asp.net and c#

for sending email applications.i am using iis6.0,window 2008 r2,what r settings requried in smtp