Web Forms :: Unable To Send A Mail Using Smtp Detail With C#?

I am not able to send a mail using my smtp detail with C#.

I am using all my smtp credentials for sending a mail but its not sending mail,displaying exception like Mailbox unavailable........

i am looking for a solution from last 3 days,

SmtpClient client = new SmtpClient();
client.Host = "mail.abc.com";
client.DeliveryMethod = SmtpDeliveryMethod.Network;
client.Credentials = new System.Net.NetworkCredential("xxxxx", "qqqq");

[Code].....

this is the code i am using.

<mailSettings>
<smtp from="sales@rrrrr.com" deliveryMethod="Network">
<network host="mail.abc.com" password="qqqq" port=25 userName="xxxxx"/>
</smtp>
</mailSettings>