Install runkit module:

wget https://github.com/downloads/zenovich/runkit/runkit-1.0.3.tgz
tar xf runkit*.tgz
cd runkit-1.?.?
phpize
./configure
make
make install

Edit php.ini:

extension=runkit.so
runkit.internal_override=1
auto_prepend=/sendmail_tracker.php

Create /sendmail_tracker.php:

touch /sendmail_tracker.php
chown root:root /sendmail_tracker.php
chmod 755 /sendmail_tracker.php

Edit /sendmail_tracker.php:

<?php

runkit_function_redefine('mail','','

        $mail_args = func_get_args();
        $sendmail_tracker_log =  "\n\n".date("d.m.y H:i:s") . "\n";
        $sendmail_tracker_log .= "### Start Mail\n";
        $sendmail_tracker_log .= "Script: ".$_SERVER[\'SCRIPT_FILENAME\']."\n\n";
        $sendmail_tracker_log .= "Mail to: ".$mail_args[0]."\n";
        $sendmail_tracker_log .= "Mail subject: ".$mail_args[1]."\n";
        $sendmail_tracker_log .= "Args: ".print_r(func_get_args(),TRUE)."\n";
        $sendmail_tracker_log .= "\n### End Mail";
        $sendmail_tracker_fp =   fopen("/var/log/sendmail_tracker_log","a+");
        fwrite( $sendmail_tracker_fp, $sendmail_tracker_log);
        fclose( $sendmail_tracker_fp );

        unset($sendmail_tracker_log);
        unset($sendmail_tracker_fp);
        unset($mail_args);
');

?>

Create logfile:

touch /var/log/sendmail_tracker_log && chmod 666 /var/log/sendmail_tracker_log

Restart Apache

Read logfile:

tail -f /var/log/sendmail_tracker_log | grep Script