How do I use sendmail on Debian/Ubuntu to send emails without SMTP or other modules?
OK, thanks to Clive I stopped assuming things and checked the php.ini file. Sure enough the sendmail_path option just had "/usr/sbin/sendmail".

I've updated php.ini with "sendmail_path = /usr/sbin/sendmail -t -i" and it's working beautifully.

Doh!