Send the form to the client's e-mail address on the textbox !

I am having problems to send a form to the client's e-mail address on the specific textbox. I can get it to the e-mail address of mine withput having trouble. I think the problem is that when I want it to send the mail to me, it is ok but when it is another user for ex. hotmail, gmal, yahoo etc. it fails. even it is a mail of another domain. For Example, name@domainname.com
My code is as shown below;
Imports System.Net.Mail Imports System.IO Imports System.Net Partial Class PDF_Form_Default Inherits System.Web.UI.Page Protected Sub Button1_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles Button1.Click SendMail(emailtxt.Text, firstname.Text) End Sub Public Function GetHtml(ByVal sUrl As String) As String Dim oWebRequest As WebRequest = System.Net.HttpWebRequest.Create(sUrl) ...