Can I really install SpamAssassin myself?

I’ve seen a number of posts from people who seem to have installed and be using their own SpamAssassin version. I’m wondering if I can do this myself but as best I can tell its utility is limited because you can only have it for a single user, and a single ftp/telnet user at that. It appears it would have no utility for mXXXXX mailbox users associated with a domain. Is this right?

You are right about mXXXXXX users; installing SpamAssassin requires an account with shell access. But you can have more than 1 shell account, and they can all share the same SpamAssassin installation (using symlinks).