I would suggest running OpenSMTPD over any of the alternatives, it's much easier to configure and should be/remain secure as it comes from the OpenBSD people (the audit from Qualys attests to that).
accept for domain "example.com" virtual <users> relay via relay.example.com
I don't see how anyone could conclude that is more difficult than postfix.PS. maybe that's one of the vulnerabilities mentioned in the report (1). Anyway it's an argument for me to run Qmail/qpsmtpd instead.
(1) https://www.qualys.com/2015/10/02/opensmtpd-audit-report.txt
I just wish we had something that wasn't written in C.