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- MacPorts installs packages under /opt/local/, has ports for Perl core and many Perl add-ons including SpamAssassin.
- Homebrew installs packages under /usr/local/, has a formula for Perl core, uses CPAN for add-ons like SpamAssassin (also compatible with Perlbrew.)
Installing SpamAssassin: Benjamin Trott of MovableType describes the process of installing SpamAssassin on OS X. It assumes that you are using a POP mail server (it could be tweaked to work IMAP, but if you're using IMAP, the popread script won't work). It also assumes that you are using a mail client that can read from mailbox-format files.
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See also: CommercialMacOSX