Adding functionality is not just a matter of adding code.
Lack of documentation is becoming a problem for acceptance.
One bug in an SMTP server can open up the whole machine for intrusion.
At the time the Sendmail program had a very poor reputation with respect to security, with four root vulnerabilities per year for two successive years.
Writing software that's safe even in the presence of bugs makes the challenge even more interesting.
The challenge with Postfix, or with any piece of software, is to update software without introducing problems.