If you're on Windows, you must use SMTP to send mail. Thus sendmail_path is ignored, because PHP does not support using sendmail to send mail on win32 (As most users won't have it installed)
Therefore you'll need to set up and correctly configure a local MTA to relay messages to wherever they need to go; this is site-specific and needs to be set right.
Talk to your network admin and ask them how to do this and what software to use. A lot of windows users seem to use the IIS SMTP server which works but is lousy at error reporting / logging (i.e. it doesn't do either).
Mark