Send Email Using Asp.net

 Hi frinds,
 i try to send email trough my application develop using asp.net2.0 and C#
i have this code on my  button click
MailMessage mail = new MailMessage();
        mail.To = "deekumavat@gmail.com";
        mail.From = "deekumavat@gmail.com";
        mail.Subject = "Sucessfull";
        mail.BodyFormat = MailFormat.Html;
        mail.Body = "Test Message";
        SmtpMail.SmtpServer = "localhost";
        SmtpMail.Send(mail); 
But it is giving me Error The server rejected one or more recipient addresses. The server response was: 550 5.7.1 Unable to relay for deekumavat@gmail.com
Please Gu ...