[Exim] SpamAssassin small problem running as nobody

I am running Debian testing with exim4-daemon-heavy as a test before
setting these up on my servers.

The integration with SpamAssassin scanning all the mail entering the
server works superbly. I am using this acl setup, amongst others:

warn message = X-Spam-Score: $spam_score ($spam_bar)
spam = nobody:true

However I get a lot of warnings in my syslog along the lines of

    "Couldn't create readable default_prefs for
        [/nonexistant/.spamassassin/user_prefs]"

I realise this is because the user "nobody" doesn't have a home
directory.

What is the most elegant solution to this problem? Is it safe to run
this as the "mail" user (whose home is /var/mail)?

Thanks for any help,
Rory
--
Rory Campbell-Lange
<rory@???>
<www.campbell-lange.net>