Where can I find either a)my SMTP log and / or b)determine/toggle whether or not SMTP is being logged at all

My server sends out emails via sendmail or SMTP, depending on the situation. I am trying to find logs for SMTP. Sources indicate that I should look at /var/log/maillog (http://ift.tt/1LK47VY, http://ift.tt/1EuRsWD)

That does indeed contain logs of everything sent over sendmail, but the SMTP messages are conspicuously absent.

When sendmail is used, it is done in a php script which just drops to shell and pipes message content to sendmail. Unfortunately, the SMTP is handled using a php class wrapper, which might account for the discrepency in logging.

Might they be being logged somewhere else? How can I determine that? Might logging for SMTP be turned off? How can I determine that, and if so turn logging on?

Using Centos 6.6 Final