Procmail versus smrsh
In http://bugzilla.spamassassin.org/show_bug.cgi?id=2393 , Jim Severino notes:
'In INSTALL, the instructions in the section "Installing SpamAssassin for Personal Use (Not System-Wide)" do not work well on systems that use smrsh (Sendmail Restricted Shell) because unless root creates a symlink to procmail in /etc/smrsh, procmail is unavailable. I think Red Hat systems use smrsh by default, so it should be pretty common.
You can find this issue raised by many people by googling on "smrsh procmail spamassassin".
In my case, we were already using procmail so I didn't even need a .forward file. I don't know if Red Hat systems use procmail by default. If this is the case, maybe you could mention that if you are on a Red Hat system, you don't need the .forward file. The docs do mention that you don't need a .forward file "if you are already using procmail", which is true, but there's no reason a Red Hat user would know that they are actually already using procmail.'
So if you are on Red Hat / Fedora Core, and run into smrsh errors about procmail not being available, you may not need the .forward file; try without it.