verifying email address

You cannot. This is not how mail protocols works. In principle, there is one way: send a mail requesting a confirmation of delivery, which is automatically sent when a user opens a mail. But first, this is not what you want, secondly, not all mail agents and especially mail clients will follow this request.

At best, you can ping the host owning the mail address. It won't tell you if the mail address itself actual or not.

—SA

SqlConnection con= new SqlConnection("DataSource:xxxxxx ......");
SqlCommand cmd=  new SqlCommand("select * from TABLE_NAME where Email = "xyz@abc.com", con);
SqlDataAdapter da= new SqlDataAdapter(cmd);
DataTable dt= new DataTable();
da.fill(dt);

if(dt.Rows.Count > 0)
{
 MessageBox.Show("email address already exists");
}

for example

Connection con;
SqlCommand cmd = new SqlCommand("select * from TABLE_NAME where email = "abc@def.com", con);
SqlDataReader reader = cmd.ExecuteReader();
if (reader.HasRows)
    {
        MessageBox.Show("email address already exists");
    }