My registration page does not like yahoo email address.

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ASP.NET

I have a registration page that a new user can setup a username and password. I also have the code to check to see if the username is in the database to see if it is an authorize user first. The user has a @yahoo.com address. When the user enters their email address every thing goes ok until the user clicks on submit. The error comes up saying, "User Name Is Not Recognized by The System!!!". Why is this happening? The user is in the database.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data.SqlClient;
using System.Configuration;
using System.Web.Security;
using System.Security.Cryptography;

public partial class SubmitPage : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        
        
        if (IsPostBack)
        {
            SqlConnection con = new SqlConnection(System.Configuration.ConfigurationManager.ConnectionStrings["HotConnectionString"].ConnectionString);
            con.Open();
            string cmdStr = "Select count(*) from Tablepass where EmailAddress='" + TextBoxEA.Text + "'";
            SqlCommand userExist = new SqlCommand(cmdStr, con);
            SqlCommand cmd = new SqlCommand("select INST_ID, EmailAddress from Tablepass", con);
            int temp = Convert.ToInt32(userExist.ExecuteScalar().ToString());
            if (temp == 1)
            {
                ScriptManager.RegisterStartupScript(this, this.GetType(), "script", "alert('User Name Already Exist!!!');", true);
                TextBoxEA.Focus();
                TextBoxEA.Text = string.Empty;
                
            }
        }
    }

    protected void Submit_Click(object sender, EventArgs e)
    {
        SqlConnection con = new SqlConnection(System.Configuration.ConfigurationManager.ConnectionStrings["HotConnectionString"].ConnectionString);
        con.Open();

        string cmdStr = "Select INST_ID, accessLevel, EmailAddress from Table1 where EmailAddress='" + TextBoxEA.Text + "'";
        string cmdStr2 = "Select INST_ID, accessLevel, EmailAddress from Table2 where EmailAddress='" + TextBoxEA.Text + "'";
        string insCmd = "Insert into Tablepass (EmailAddress, Password, INST_ID, accessLevel) values (@EmailAddress, @Password, @INST_ID, @accessLevel)";
        string insCmd2 = "Insert into Tablepass (EmailAddress, Password, INST_ID, accessLevel) values (@EmailAddress, @Password, @INST_ID, @accessLevel)";

        SqlCommand insertUser = new SqlCommand(insCmd, con);
        SqlCommand insertUser2 = new SqlCommand(insCmd2, con);

        insertUser.Parameters.AddWithValue("@EmailAddress", TextBoxEA.Text);
        insertUser.Parameters.AddWithValue("@Password", TextBoxPW.Text);
        insertUser.Parameters.AddWithValue("@INST_ID", TextBoxINST_ID.Text);
        insertUser.Parameters.AddWithValue("@accessLevel", TextBoxaccessLevel.Text);

        try
        {
            insertUser.ExecuteNonQuery();
            con.Close();
            Response.Redirect("Login.aspx");
        }
        catch (Exception er)
        {
            ScriptManager.RegisterStartupScript(this, this.GetType(), "script", "alert('User Name Is Not Recognized by The System!!!');", true);
        }
        finally
        {
        }
    }

    protected void TextBoxEA_TextChanged(object sender, EventArgs e)
    {

        using (SqlConnection con = new SqlConnection(System.Configuration.ConfigurationManager.ConnectionStrings["HotConnectionString"].ConnectionString))
        {
            con.Open();

            SqlCommand scmd = new SqlCommand("Select INST_ID, EmailAddress, accessLevel from Table1 where EmailAddress = @TextBoxEA", con);
            SqlCommand scmd2 = new SqlCommand("Select INST_ID, EmailAddress, accessLevel from Table2 where EmailAddress = @TextBoxEA", con);

            scmd.Parameters.Add(new SqlParameter("@TextBoxEA", TextBoxEA.Text));
            scmd2.Parameters.Add(new SqlParameter("@TextBoxEA", TextBoxEA.Text));

            TextBoxINST_ID.Text = string.Empty;
            TextBoxaccessLevel.Text = string.Empty;

            using (SqlDataReader dr = scmd.ExecuteReader())
            {
                while (dr.Read())
                {
                    TextBoxINST_ID.Text = dr["INST_ID"].ToString();
                    TextBoxaccessLevel.Text = dr["accessLevel"].ToString();
                    
                }
            }

            using (SqlDataReader dr2 = scmd2.ExecuteReader())
            {
                while (dr2.Read())
                {
                    TextBoxINST_ID.Text = dr2["INST_ID"].ToString();
                    TextBoxaccessLevel.Text = dr2["accessLevel"].ToString();
                    
                }
            }

            }
        }
    }