AFAIK,,, this error message is displayed when a message is sent and while it is delieverd to the recepient it reaches a point that it is not able to connect to the recepient domino server .
Does this message disply when you send a message or when somebody sends you a message ?????
Any why,,, in both cases you have to make sure on the recepient servers there is no AntiSpam checklist is active .
Or the user you this email is sent to is really available ( may be he resigned and his email was deleted !!!!)