Unable to send e-mail to a specific Domain name

I’m using Exchnage 2000 and My Boss has been able to forward E-mails to his private business account. this past 10 days no e-mails he forwarded has been delievered to his private e-mail. i checked and there is 5 of them sitting on the smtp queue waiting to be retried. he receives e-mails from all other places except from our Domain.
my question is, how would i go about tracing this, or get an answer why the delivery was delayed.

any help is greatly appreciated.

thanks.