I have an SMTP communication problem .

I am getting this error when sending to a specific email address . I use Exchange 2003.I only have this problem with this address and have tried sending it from a hotmail account which works fine. Any ideas ?

Your message did not reach some or all of the intended recipients.

      Subject:      Help
      Sent:      16/04/2009 14:22

The following recipient(s) cannot be reached:
    There was a SMTP communication problem with the recipient's email server.  Please contact your system administrator.
            <"mydomain.co.uk" #5.5.0 smtp;550 Submission from dynamic "my IP Address "requires authentication>

Who is Participating?
The error is coming from the remote server, so unless you route your email through another server the problem is with the other side. It is not an Exchange message.

The remote site thinks you are on a dynamic IP address. Are you? If so then you will need to use an SMTP connector to route email for that domain and others via your ISPs SMTP Server.

It could also be that the remote domain isn't setup correctly and is pointing their MX records at the wrong host.

johnkcarsonAuthor Commented:
No we are on a fixed IP. The recipient says they have no problem getting mail from anyone else. We use reverse DNS lookup and our public ip address is in the MX record for our domain
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