One more item with regards to use with windows.
While trying to use formmail.php shareware I would also receive the failure message. I discovered that if I substitute backslash r in place of backslash n (and made sure there were no backslash n in the valves it pasts to mail() then it would work on windows.
It also somewhat worked on my UNIX ISP, except that the message headers were not totally visible using Outlook and the message was "sent on behalf of" instead of lust listing the user email address.
This is unfortunate that there is this difference between windows and unix -- not sure how to work around it without lots of extra programming.
Statements from formmail (where there is a \r there was a \r\n):
// mail the content we figure out in the following steps
function mail_it($content, $subject, $email, $recipient) {
mail($recipient, $subject, $content, "From: $email\rReply-To: $email\rX-Mailer: DT_formmail");
}