To install SpamAssassin under Debian, do:
apt-get install spamassassin procmail
To enable SpamAssassin, a .procmailrc needs to be created in the home directory:
MAILDIR=$HOME/mail
# call spamassassin
:0fw
| /usr/bin/spamassassin
:0:
* ^X-Spam-Status: Yes
spam
SpamAssassin includes a Bayesian learning filter, so it is worthwhile training SpamAssassin with your collection of non-spam and spam, if possible:
sa-learn --spam --mbox ~/mail/folder
sa-learn --ham --mbox ~/mail/folder