Amavis gives you an interface between mail servers (MTAs such as Postfix or DoveCot) and mail filters (ClamAV, SpamAssassin). In many cases it is more efficient than running separate daemons like spamd.
BuildAUR from the AUR and install it with pacman.
If your hostname is not a FQDN, you must set $myhostname in /etc/amavisd/amavisd.conf. You probably want to set $mydomain too.