Send email with multiple/defined “To” recipients

I have a kind of form in asp.net. I want to send the form to email depending upon which city is selected from menu choice.
Here's the code in cs file:

htbl["@HtmlPage"] = sbTableData.ToString();
    string _From = ConfigurationManager.AppSettings["MailFrom"].ToString();

      string _to = string.Empty;

switch(ddlRegion.SelectedItem.Value)
{
case "1": _to = ConfigurationManager.AppSettings["MailToA"]; break;
case "2": _to = ConfigurationManager.AppSettings["MailToB"]; break;
case "3": _to = ConfigurationManager.AppSettings["MailToC"]; break;
case "4": _to = ConfigurationManager.AppSettings["MailToD"]; break;
}

and in the webconfig file I have:

At present which is working-

<appSettings>
    <add key="SMTP_HOST_ADDRESS" value="mail.company.com"></add>
    <add key="CREDENTIAL_USER" value="yourcredentialuser"></add>
    <add key="CREDENTIAL_PASSWORD" value="credentialpassword"></add>
    <add key="MailFrom" value="Admin@company.com"></add>
    <add key="MailTo" value="admin1@company.com"></add>



  </appSettings>

After I changed the webconfig file

<appSettings>
    <add key="SMTP_HOST_ADDRESS" value="mail.company.com"></add>
    <add key="CREDENTIAL_USER" value="yourcredentialuser"></add>
    <add key="CREDENTIAL_PASSWORD" value="credentialpassword"></add>
    <add key="MailTo_A" value="A@company.com"></add>
    <add key="MailTo_B" value="B@company.com"></add>
    <add key="MailTo_C" value="C@company.com"></add>
    <add key="MailTo_D" value="D@company.com"></add>
    <add key="MailFrom" value="Admin@company.com"></add>


  </appSettings>

Now when I click on submit on the form I am getting an error message
Object reference not set to an instance of an object.

If you require any more info please let me know.

Tags: C#, ASP.NET