I can't start spamassassin service

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.