I run a forum and a member emailed me saying they didn't receive an email to validate their account - it is set up correctly and I know emails get sent out to the majority of people. I emailed him back and I am getting the following mail delivery failed message so there is a problem for me to send to this account.
I doesn't provide a reason for the email not being delivered - except that it failed! The information that you've obfuscated might provide a clue, but the way you've changed it makes it impossible to infer anything from it. You're better posting the exact headers. If you want to hide an IP then put in something like 99.99.99.99 and for "myhost" or "mycomputername" replace them with similar word. e.g. if the "myhost" part actually says mail.myhost.com then replace it with something like mail.hostname.com, not just "myhost". If you do that, and repost it, it might provide a clue as the exact format of these things are important when it comes to email delivery. Then again, it might not tell us anything. If you can't do that, then the only way to know for sure is for the recipient to ask his email provider why the email was rejected. To do that he should tell them the date and approximate time of the email, who it was from and who it was being sent to. That way they may check the logs for him - but don't be surprised if they want to charge for the support time.
Looks like you are using Exim... grep your log files and see if it provide more details on the error... as what you have posted above is a very generic error message and could be lot of things.