Killing the INSTALL file.
It hasn't had significant updates in almost 10 years. The package versions referred to in it are now long since obsolete, and the installation method might be outdated as well.
Generally, it's not really our duty to provide manuals on installing everything from Apache and PHP to Exim to ClamAV and SpamAssassin, nor do we have the manpower to do that. The users should refer to the manuals provided by their OS distributions or similar sources instead.