After the upgrade of Wheezy to Jessie, Spamassassin is not added to the startup services. Hence if you were using it before in your mail setup, you will run into the following error in /var/log/syslog
spamc: connect to spamd on ::1 failed, retrying (#1 of 3): Connection refused
spamc: connect to spamd on 127.0.0.1 failed, retrying (#1 of 3): Connection refused
Debian has a similar bug filed (#764438) even after a fresh reinstall.
So what is the solution
Simply activate Spamassassin via systemctl enable spamassassin