I have just upgraded a server from Ubuntu 14.04 LTS to 16.04 LTS.
My issue is that now upon reboot, neither Postfix nor Spamassassin are running.
There are init.d scripts for both these services and the services do start if I manually run commands like
sudo service postfix start
sudo /etc/init.d/postfix
sudo service spamassassin start
sudo /etc/init.d/spamassassin
and there are all the appropriate symlinks from /etc/rc[0123456].d/ to the init.d scripts.
After boot, there are no 'postfix', 'spamassassin' or 'spamd' entries in /var/log/mail.log or /var/log/syslog until I manually start Postfix and Spamassassin.
Update (2016_09_04): It seems that Postfix does start eventually (after 4 or 5 minutes) if not immediately. However, even 45 minutes after a reboot, spamd is still not running but starts immediately following 'sudo service spamassassin start'.