0x800CCC6A // The server responded: 451

Hi,

I ran into an issue that I don't how to fix.

I can send/ receive newsletters and everything seems to work fine but when I try to forward the message (generated by simplenews), I can't... I get the following error:

"Task 'admin@mywebsite.com - Sending' reported error (0x800CCC6A) : 'Your outgoing (SMTP) e-mail server has reported an internal error. If you continue to receive this message, contact your server administrator or Internet service provider (ISP).  The server responded: 451 See http://pobox.com/~djb/docs/smtplf.html.'"

Here are some specs:

VERSION / MODULES (in question)
--------------------------------
- D6.14
- Simplenews 6.x-1.0-rc6
- Simplenews on register 6.x-1.0
- Mime Mail 6.x-1.x-dev

I tried the following modification in the file /includes/mail.inc with no success...

 return mail(
      $message['to'],
      mime_header_encode($message['subject']),
      // Note: e-mail uses CRLF for line-endings, but PHP's API requires LF.
      // They will appear correctly in the actual e-mail that is sent.
      str_replace("\r\n", '', $message['body']),
      // For headers, PHP's API suggests that we use CRLF normally,
      // but some MTAs incorrecly replace LF with CRLF. See #234403.
      join("\r\n", $mimeheaders)
    );

EMAIL CLIENT
--------------------------------
- Outlook 2007

PHP VERSION
--------------------------------
- 5.1.6

Server
--------------------------------
- CentOS 5 with Plesk 8.6 (32-bit) -> Qmail

Any suggestion would be appreciated.

Thanks!