This is kind of a subjective thing, but you can install postfix, just set it up to accept only incoming mail from your localhost or local IP. That will in turn try to deliver mail to external sites from your local scripts and applications.
Once installed, you edit your
/etc/postfix/main.cf file so the
inet_interfaces is set to
127.0.0.1. Or you can bind it to your public IP and the localhost. Then restart postfix.
You also want to check the
mynetworks = 127.0.0.0/8,
smtpd_recipient_restrictions = permit_mynetworks. These may need to be tuned to your particular configuration, but these settings can be tweaked to only accept from a particular IP or set of IPs, depending on how your server is seen by postfix when your applications connect.