What simpler mail server can I use instead of postfix?

A simple relay-only mail transport agent is nullmailer.

There is a good configuration guide here: http://www.troubleshooters.com/linux/nullmailer/

That configuration guide is old enough that it omits one important configuration file: /etc/nullmailer/allmailfrom. Put just the email address you want the mail to be sent From in there. This helps tremendously when configuring various applications to send mail.