RCPT TO problem using relay host

[Dennis Putnam]

I have a machine running postfix that is required to relay mail  
through my ISP's mail server. When I try to send mail, I get this error:

550 [PERMFAIL] destination not valid within DNS (in reply to RCPT TO  
command)

Can someone explain what this error means? If I use 'telnet' to send  
the RCTP TO command manually it accepts it.

Thanks.