Have a look on the MM exchange for a tag called 'CF_ResendUndeliverableEmail'. I haven't used it my self, but have seen people rave about it on here in the past.
Failing that you can do it manually by looking at your mail.log file in the coldfusion install directory. Also the undeliveriable emails will be in the coldfusion install directory under mail\underliveriable
MX this is in the logs directory, can't remember where it is in cf 4, but i think it was in the the mail directory
Hope this helps!
Tony