Hello:

I have an Exchange 2007 working fine, and it’s possible to send and receive
mails from mail clients to external domains.

I try to send a mail from my application using this server. Te code is like
this:

  MailAddress From = new MailAddress("sen***@mydomain.com", "Automated
Sender");
  MailAddress To = new MailAddress(***@externaldomain.com");
  MailMessage myMail = new MailMessage(From, To);
  myMail.IsBodyHtml = true;
  myMail.Subject = "Automated mail";
  myMail.Body = "Test mail...";
  myMail.Sender = From;
  SmtpClient smtp = new SmtpClient("mailserver.mydomain.com");
  smtp.UseDefaultCredentials = false;
  smtp.Credentials=new System.Net.NetworkCredential("username","password");
  smtp.Send(myMail);

It works if the destination domain is an internal domain, but with and
external domain it launch an "Unable to relay" exception:

  "El buzón de correo no está disponible. La respuesta del servidor fue:
5.7.1 Unable to relay"

English, aprox. translate:

  "Mailbox unavailable. Response from server: 5.7.1 Unable to relay"

If I try to send a mail from Outlook with the authentication option active
IT WORKS!

This source code works with Exchange 2003.

Some ideas?
Thank's and sorry for my bad English!

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