"Client was not authenticated" error

[Silmarillion]

We are running Exchange 2000 internally on Windows 2000 server. We use Outlook 2000 for our mail client. I have one user who is getting intermittent bounce messages when trying to send to one particular external e-mail address. The error message is as below:

From: System Administrator
Sent: Thursday, June 19, 2003 12:55 PM
To: 'John Doe'
Subject: Undeliverable: Message Subject

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

Subject: Message Subject
Sent: 6/19/2003 12:55 PM

The following recipient(s) could not be reached:

'John Doe' on 6/19/2003 12:55 PM
The recipient could not be processed because it would violate the security policy in force
<server.domain #5.7.3 smtp;505 5.7.3 Client was not authenticated>

All I can find on this refers to clients like Outlook Express trying unsuccesfully to authenticate to the server to send, but this is not our situation.

Facts:

1) The message will occasionally go through. I have got the bounce message as postmaster, and done a resend, and it has been fine.

2) The error is only occurring with one user. I have never seen this error message before.

3) The error is only occuring to one e-mail address, and then only intermittently. I have not tested other addresses at the same domain.

Any ideas? It looks like the message is being generated at our end, but why?

 

[Silmarillion]

A bit of further information - I just talked to the IT person at the company we are trying to send to, and they do not run their own mail server. All their clients run POP3 for e-mail, and their domain mail is hosted by their ISP (Telus). They are also our ISP, so I will continue to pursue it with them, but I do not have much hope of them being even remotely helpful, based on past experience.