How does cron know what MTA to use?

How does cron know what MTA to use? Where is this info stored please? (Using Ubuntu KK.) Edit: Here's the whole story: I enabled the MAILTO option in crontab, and scheduled some task. When the task got triggered, the system complained that there is no MTA installed, so I installed ssmtp. Interestingly, I never installed sendmail on my system, nor do I think it is installed by default. Hence all I have is ssmtp. Now, I need to make sure that whatever application does the emailing is bound to some specific NIC, hence it needs to be launched prefixed by firejail. The question is now, how do I know what email application is launched by cron, and how can I launch it with the firejail prefix.