Amavis
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.
Contents
Install & Setup
Install amavisd-new from the AUR. You'd be wise to also install optdepends such as p7zip and unrar so your filters can actually see inside compressed files.
If your hostname is not a FQDN, you must set $myhostname in /etc/amavisd/amavisd.conf. You probably want to set $mydomain too.
After that, you can start the amavisd service with systemctl and possibly enable it.
Some ways to check for errors:
systemctl status amavisd journalctl -xbo short -u amavisd
Postfix
Digest of the excellent upstream README.