title: Email Form Stopped Working Suddenly? When its been working fine?
body: <p>I have an enquiry form on a site, which has been working for a while and yesterday just stopped working - I have got the error message and never seen it before?</p>
  
  <pre><code>Service not available, closing transmission channel. The server response was: dns lookup failed for sender domain - System
  
  at System.Net.Mail.MailCommand.CheckResponse(SmtpStatusCode statusCode, String response)
  at System.Net.Mail.MailCommand.Send(SmtpConnection conn, Byte[] command, String from)
  at System.Net.Mail.SmtpTransport.SendMail(MailAddress sender, MailAddressCollection recipients, String deliveryNotify, SmtpFailedRecipientException&amp; exception)
  at System.Net.Mail.SmtpClient.Send(MailMessage message)
  at general.SendMail(String strFrom, String strTo, String strSubject, String strMessage)
  at enquire.ReserveButton_Click(Object sender, EventArgs e)
  at System.Web.UI.WebControls.Button.OnClick(EventArgs e)
  at System.Web.UI.WebControls.Button.RaisePostBackEvent(String eventArgument)
  at System.Web.UI.WebControls.Button.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String eventArgument)
  at System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument)
  at System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData)
  at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
  On page enquire.aspx 
  </code></pre>
  
  <p>Any ideas greatly appreciated?</p>