emails not reaching its targets

I have an exchange 5.0, emails to 3 different targets are not getting there, but we are getting relay confirmations but they actually never reach the target, this is happening with just 3 different targets, I checked on my eschange server and every thing is working normally, I dont understand why this is happening.
please comment on this issue, I dont know where else to check.
please help!
this is the confirmation I get from my server;
“This is an automatically generated Delivery Status Notification.

Your message has been successfully relayed to the following recipients, but the requested delivery status notifications may not be generated by the destination.

user@domain.net
Thanks!