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Editorial Team
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Asked: 2026-05-12T00:08:02+00:00

I have an enquiry form on a site, which has been working for a

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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?

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& 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

Any ideas greatly appreciated?

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1 Answer

  1. Editorial Team

    It looks like the SMTP server is trying to do a lookup of the sender domain (your From address, perhaps?) and is failing. This may have started happening due to a failure of something on the SMTP server, or it could have been a change in the policy of the SMTP server.

    If you manage your own SMTP server, then that is where you may need to look for answers. If you are sending through another SMTP server (such as one at your ISP) you may need to ask the administrator of that server for assistance.

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