5.7.1 error on Exchange 2000

I have two regular users that are trying to send an e-mail to a person outside of the office and they are getting this error seen below…

You do not have permission to send to this recipient. For assistance, contact your system administrator.

They have tried multiple times and continue to get this error. BUT when I try (the Administrator) I can send to this outside person just fine. Is this something wrong on my end or their end? If it’s on my end where can I fix this.

So far everythin I’ve found on MS site has to do with everyone getting this error. HELP. thanks