did you have a look at the postfix logs what they say?
my php.ini does not have any mail thing attached:
cp:/backup/old# cat /etc/php5/apache2/php.ini | grep mail
; Define the anonymous ftp password (your email address)
;sendmail_from = firstname.lastname@example.org
; For Unix only. You may supply arguments as well (default: "sendmail -t -i").
; to the sendmail binary. These parameters will always replace the value of
; the 5th parameter to mail(), even in safe mode.
; to other person via. email/irc/etc.
; mail(), ereg(), etc are overloaded by mb_send_mail(), mb_ereg(),
; 1: Overload mail() function