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. You'd be wise to also install optdepends such as and so your filters can actually see inside compressed files.
pacman -Syu amavisd-new p7zip unrar
If your hostname is not a FQDN, you must set $myhostname in /etc/amavisd/amavisd.conf. You probably want to set $mydomain too.