Emails bounce back – SMTP errors

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    Hi all,

    We have Exchange 2003 and we could send/receive emails outside our domain without problems.

    Yesterday we had problems with our internet link, and our ISP fixed it, but after that we can receive emails from outside/inside network and only send internally, but not to outside domains (, etc).

    Here’s the smtp log:

    This is an SMTP protocol error log for virtual server ID 1, connection #8.
    The remote host “xx.xx.xx.xx”, responded to the SMTP command “rcpt” with “550-Please turn on SMTP Authentication in your mail client. ( [xxx.xx.xx.xx]:5666 is 550 not permitted to relay through this server without authentication. “.
    The full command sent was “RCPT TO: “. This will probably cause the connection to fail.


    A non-delivery report with a status code of 5.5.0 was generated for recipient rfc822;
    [email protected] (Message-ID ).
    Cause: This message indicates a generic protocol error (SMTP error). For example, the remote SMTP responds to an issued EHLO with a 500 level error and the sending system will QUIT the connection and report this with NDR indicating the remote SMTP server canÆt handle the protocol.
    Solution: View the SMTP log or run a netmon trace to see why the remote SMTP server rejects the protocol request.

    Any help?

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