SpamAssassin's integration with Exim4 is currently broken on my mail server. I need to keep receiving mail, so I removed the router and transport from the Exim4 configuration, and then pondered how best to use Evolution and Spamassassin themselves to filter spam.
I found this marvellously concise and accurate guide:
[link|http://cgi.timandkathy.co.uk/blog/archives/computing_internet/evolution_spam_filtering_with_spamassassin.php|http://cgi.timandkat..._spamassassin.php]
Enjoy!