Unable to send mail to my ISP mailbox.

[Boney]

I am trying to use Postfix to send mail via ( or to ) my ISP.
I have created a small PHP script which uses the internal 'mail($to, $subject, "", $header)' function, where the $header contains ( amongst other things ) 'From' and 'Reply-To' as a valid email address at my ISP.
The message is being rejected every time with:-

Final-Recipient: rfc822; to_me@my_isp.com
Action: failed
Status: 5.0.0
Diagnostic-Code: X-Postfix; host mx.core.my_isp[nn.nn.nn.nn] said:
550-Verification failed for <apache@localhost.localhost> 550-Unrouteable
address 550 Sender verify failed (in reply to RCPT TO command)

How to I configure postfix to advertise as 'a_user@my_isp.com' instead of 'apache@localhost....' ? or, alternatively, not send the RCPT TO.

Mandrake 10.0

Please, please help

[the_unforgiven]

That is because your postfix is not configured for relaying...
Read the SMTP RFC (ftp://ftp.rfc-editor.org/in-notes/rfc2821.txt) - Section 4.1.1.3