Exchange 5.0

Have a customer running Exchange 5.0 on an NT 4.0 (SP 6) box. Clients are running outlook 2000. They cannot send emails to a couple of domains. For example they can send mail to most addresses but when they try to send mail to someone at verizon.net or stargate.net, the emails come back as undeliverable. It’s only a handful of domains that they cxant send mail to. Can’t find anything in the Exchange settings that would stop the mail from being sent to those domains. They are on a DSL connection and their (ISP’s) DNS servers are resolving names as far as I can tell. Anyone have any idea of what could be going on? Any help would be appreciated. Thanks.