I have some VB code the sends an email (from web server): Dim smtp As New SmtpClient("m04.internetmailserver.net") smtp.UseDefaultCredentials = False smtp.Credentials = New System.Net.NetworkCredential("...", "...") I sometimes get a returned email stating: The mail system ... naver.com Sender rejected because SPF check failed Wc-2-pRlTcy-1qjPHnnAxw - nsmtp (in reply to MAIL FROM command) Is this due to a problem on the Winhost side? Anything I can do? I only have this problem when sending to naver.com emails.