macOS 10.12 "Sierra" Postfix Launch Daemon Property List

org.postfix.master.plist

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
  <dict>
    <key>Label</key>
    <string>org.postfix.master</string>
    <key>Program</key>
    <string>/usr/libexec/postfix/master</string>
    <key>ProgramArguments</key>
    <array>
      <string>master</string>
      <string>-e</string>
      <string>60</string>
    </array>
    <key>QueueDirectories</key>
    <array>
      <string>/var/spool/postfix/maildrop</string>
    </array>
    <key>AbandonProcessGroup</key>
    <true/>
    <key>RunAtLoad</key>
    <true/>
    <key>KeepAlive</key>
    <true/>
  </dict>
</plist>

Copy this file to /Library/LaunchDaemons/org.postfix.master.plist in order for Postfix daemon to run automatically on boot.