PHP mail.log not registering PHP mail() sent emails

[Tantrumedia]

My System: Plesk 12.0.18 & CentOS 6.6 with PHP 5.3.3-46.el6_6

I changed the following in /etc/php.ini and restarted Apache;
; Log all mail() calls including the full path of the script, line #, to address and headers
mail.log = /var/log/php_sent_mail.log
This should log all PHP mail() usage across all domains to the log file, but it didn't.

I therefore logged into Plesk and added an Additional Configuration Directive to tantrumedia.com;
mail.log = /var/log/php_sent_mail.log
I checked, and this was indeed added /var/www/vhosts/system/tantrumedia.com/etc/php.ini
I restarted Apache, and tried sending another email using PHP mail() from mydomain.com.
Again - nothing appears in /var/log/php_sent_mail.log

Is Plesk blocking this? Does it have anything to do with my Postfix (Version:2.8.17-14042513)?

Any help appreciated!