Can you provide a sample script?

Well it's pretty straight forward:

$mailrecipient="davidfelton@codemasters.com";
$mailsubject="A message from your friend";
$mailmessage="Hello, this is an email sent with PHP";
$mailheaders="From: Dave <dave@davidfelton.co.uk>";

mail($mailrecipient,$mailsubject,$mailmessage,$mailheaders);

What are your INI-settings about mail? I.e., your smtp-server, etc...

smtp server is set to be localhost, which works fine with php 4.0.6

So, any other ideas?

i have the same problme with win nt 4.0 terminal edition.
did you find a solution...?
it seems like it can't find the ini file.
thanks

If your php can't read the ini file (patch of file in phpinfo shows up as c:/winnt/ instead of c:/winnt/php.ini) You need to make sure that php has access to read the php.ini file through your security settings.
My solution to the problem was to replace all the calls to mail() with my own function that writes out the raw emails to the mail pickup folder in c:\inetpub\mailroot\
 - it took ages!

I got the same error on Win2000+IIS5. I used this on the php page to overwrite ini values:
<?php
ini_set("sendmail_from",NULL);
ini_set("sendmail_path",NULL);
ini_set("SMTP","aaaa.xxxx.cccc.gggg");// Add SMTP server IP
mail($mail_to, $mail_subject, $mail_body, $mail_headers);
?>
It seems you have to explicitly tell PHP that Sendmail is not in the server :)

SMTP error reporting has been improved, please try a snapshot from http://snaps.php.net/win32/php4-win32-latest.zip

No feedback was provided for this bug for over a month, so it is
being suspended automatically. If you are able to provide the
information that was originally requested, please do so and change
the status of the bug back to "Open".

Yes, I met the same problem in php server. I use php 4.1.2 for windows binary. And when I use mail() function the error message "unknown error" is issued. I don't know why this action can cause the error. In the php.ini I set like the following:
[mail function]
; For Win32 only.
SMTP = smtp.ericsson.se

; For Win32 only.
; sendmail_from = 

; For Unix only.  You may supply arguments as well (default: 'sendmail -t -i').
;sendmail_path =

I can not find any faulty setting in this configuation file. Is there anybody can give me a hand?