Unknown error

[Anon]

hi.
I have IIS 4.0 on win NT 4.0 and I want to use mail function,but it generates error message :

"Unknown error in sendmail.php on line 13"

and line 13 is:

mail("naderi@afranet.com","test","test");

I set my php.ini file correctly.

why it happens?
please help.
bye.