Check mail logs

I recently sent out lots of e-mails, and apparently ~200 of them failed. What is the location to the mail log(s) so that I can verify the remaining e-mails were indeed sent, and find out what the error was?

Edit: I found maillog in /var/log ... I used a PHP script that sent through SMTP (Through PHPMailer class), but shouldn't the info still be in maillog? When I do tail maillog, the last entry is from April 18...