Exchange 5.5 undeliverable mails

I have Internet Mail service running on exchange 5.5 server with 30-40 users all on NT4 workstation. While all the users have no problem in receiving mails (POP) there are some users who get an undeliverable mail message from System Administrator stating “The following receipent(s) could not be reached:’ followed by the email id as mentioned in the TO/CC/BCC field when sending out mails(SMTP).

I will appreciate if someone would advise where I may be going wrong.

Thanks & regards
K.S.Powar