I can't start spamassassin service
Solution 1:
Run sudo netstat -lptn | grep 783 and get the PID (right most side of the output - some thing like 2332/spamd.pid ) of the program that uses 783 port. Then kill that program with sudo kill -9 PID.
Then start the spamassasin
service spamassassin start
If you still get the INET6 socket error, try running spamd without IPv6 support (assuming you don't need it).
sudo nano /etc/default/spamassassin
Add -4 to your OPTIONS line
spamd --help
...
-4, --ipv4-only, --ipv4 Use IPv4 where applicable, disables IPv6
...
Otherwise, troubleshoot your IPv6 config.