Mail function problem

Hi,

Trying to use aformmail on IIS, but getting the following error.

The mail function works normally, but when using the code in aformmail:
mail($_send_to, $_subject, $mail,
"Mime-Version: 1.0\r\nFrom: $_send_from\r\nContent-Type: multipart/mixed;\n boundary=\"$mime_delimiter\"\r\nContent-Disposition: inline");

This produces the error -

Warning: mail() [function.mail]: SMTP server response: 501 5.5.4 Invalid Address in D:\Inetpub\wwwroot\SIAweb2005\a_form_send.php on line 221

Warning: Cannot modify header information - headers already sent by (output started at D:\Inetpub\wwwroot\SIAweb2005\a_form_send.php:221) in D:\Inetpub\wwwroot\SIAweb2005\a_form_send.php on line 68

Any ideas appreciated.

Thanks