mail function

[Anon]

I am trying to send mail externily from my SMTP server(exchange 5.5) using mail() function.

I have no problem if I send to a internal address , however if I try to sent to a external address ([e-mail]) I recieve Error Message: From my exchange Server to my Exchange client "The following recipient(s) could not be reached:" from exchange system administartor.And mail is not sent. But In the exchange client I resend this undeliverable mesaage is is sent without error. Please help.

Thanks

Our configuration

---

PHP4.03
Exchange 5.5 server
IIS 5.0