Let's assume CustomBuild is installed.
We have to configure our options.conf and enable the spamassassin installation.
cd /usr/local/directadmin/custombuild vi options.conf
Now we enable it under #mail options and save:
#Mail options exim=yes eximconf=no clamav=no spamassassin=yes dovecot=yes pigeonhole=no
Next we run update custombuild
BUT before we run ./build spamassassin we must first download all the requirements (as custombuild will not do this for us):
yum install spamassassin -y; yum -y erase spamassassin
So now that yum downloaded all the requirements we can now do the install. Though let's also update exim before we do this:
./build exim ; ./build spamassassin
And that should do it.