Do they just have one MX record in their DNS? The error is typical of one generated by a spam-filtering service--either a separate gateway or a filtering device. If they have it set up this way, it is possible that sometimes the message hits their main SMTP gateway and gets through fine and sometimes it gets caught in the filter.
Reasons? Verify that YOUR mail server isn't in any blacklists and that you have full forward and reverse DNS for your mail server's IP - http://www.dnsstuff.com is a good place to check.
If your server passes all their tests, then contact the server admin for the external domain and have them check their log files for indications that their server is refusing the message and why--if the error is getting generated on that server, chances are it is getting logged there as well:)
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by: SembeePosted on 2004-12-14 at 10:40:05ID: 12822431
Go to dnsreport.com and enter the domain name that is causing you problems. This will test all settings that the domain has.
My first instinct is that the remote domain may have mutiple MX records and one of those could be wrong. When your server tries to deliver to the other MX record the message bounces.
The administrator of the other server needs to take this on as the issue is almost certainly at his end... if he claims there are no problems then ask why his server is generating the message.
Simon.