Illegal character(s) in domain string

I have a problem with messages delivered to a Domino server (R5 or R6.5) via IMSS for SMTP 5.15 server where the recipient address is not listed in the Domino Directory. When Domino attempts to send a NDR back via IMSS, IMSS rejects the message due to “illegal character(s) in domain string”.

Any one know how I can resolve this issue?

This happens even if the original senders address is valid. It would appear that the generated NDR has a blank senders address and IMSS is therefore rejecting the message due to it not adhering to RFC 822.

Subject: illegal character(s) in domain string

read RFC 821NDR may be sent from null address