Checking for invalid characters in string

Below is the code on my Handler page for a web form. How can I check the non-email fields to make sure they do not contain an '@'. The incoming fields/keys I want to check are name, address, company. Thanks.

<script language="C#" runat="server">

public StringBuilder sb;
private void Page_Load(object sender, System.EventArgs e)
      Response.CacheControl = "no-cache";
      Response.Expires = -1;

      string sFrom = "";
      sb = new StringBuilder("<table border=\"0\">");

      // get values from post
      NameValueCollection coll = Request.Form;
      String[] arr1 = coll.AllKeys;

      for (int i = 0; i < arr1.Length; i++)
            string key = arr1[i];
            // prevent script insertion
            string val = coll[key].ToString().Trim().Replace("<"," ").Replace(">"," ");
            if (key == "email" && val != "")
            sFrom = val;
            sb.Append("<tr><td>" + key + ":</td><td>" + val + "</td></tr>");
      if (sFrom == "")
      MailMessage mail = new MailMessage();
      mail.To = "";
      mail.From = sFrom;
      mail.Subject = "Website Contact";
      mail.BodyFormat = MailFormat.Html;
      mail.Body = "<html><body>" + sb.ToString() + "</body></html>";
//      mail.Fields.Add("", "1");
//      mail.Fields.Add("", "");
//      mail.Fields.Add("", "");
      SmtpMail.SmtpServer = "EXCH.DOMAIN.LOCAL";
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            string name;
           if( name.Contains("@"))
{"name contains @");

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