title: Mailbox unavailable. The server response was: 5.7.1 Unable to relay for abc@gmail.com
body: <p>I am using the below code;
  One of the following points could be the reason of the error, but I don't know;</p>
  
  <p>1-The application is on development machine
  2-The smtp ip is hosted on the server while app is running on localhost</p>
  
  <p>I am getting this error "Mailbox unavailable. The server response was: 5.7.1 Unable to relay for mhtbusoum@gmail.com";</p>
  
  <pre><code> Dim message As New MailMessage
   message.From = New MailAddress(Strings.LCase("mhtbusoum@mysite.com"))
   message.To.Add("mhtbusoum@gmail.com")
   message.IsBodyHtml = True
   message.Priority = MailPriority.Normal
   message.Subject = ("Testing SmtpClient email sending")
   message.Body = ("It is to test the email sending without any password")
   Dim client As New SmtpClient("mailSending.actualSite.com", 25)
   client.UseDefaultCredentials = False
   client.Send(message)
  </code></pre>