Here ya go:

The following recipient(s) could not be reached:

'User Name' on 6/7/2006 9:19 AM
No transport provider was available for delivery to this recipient.
Evidently the user authentication doesn't produce an error, it just pops up repeatedly...he eventually then gets the above bounce back message.

So if the guy is using a different ISP to get on the internet...then try to send mail through the mail servers...that reside at another ISP....it will probably fail....as the mail server will think he is trying to relay???
In some cases with xDSL and Leased line, you have to use the SMTP server of the ISP server not your mail service's provider .... not sure if this applies to your case ... usually you get a "Relaying Denied 550" message .... try to create a new account ....
I suspected that too, but if that were the case, wouldn't the problem then be consistent? The thing is it does allow email through at other times...