Running spamc via postfix not as user "nobody"

[Michael Frotscher <in...@tronserver.dyndns.org>]

Hello, all

I'm trying to get spamassassin to recognize my ~/.spamassassin/user_prefs 
files. I'm using spamc/spamd, which means I need to run spamc as the user the 
mail is directed to. Obviously spamc is called as the user nobody:

spamd[28313]: info: setuid to nobody succeeded
spamd[28313]: Creating default_prefs [/nonexistent/.spamassassin/user_prefs]
spamd[28313]: Cannot write to /nonexistent/.spamassassin/user_prefs: No such 
file or directory
spamd[28313]: Couldn't create readable default_prefs for 
[/nonexistent/.spamassassin/user_prefs]

spamc is called as a content-filter from within postifix which requires a 
username - my master.cf currently reads as follows:

spamassassin    unix     -      n       n       -       -       pipe
   user=nobody argv=/usr/bin/spamc -e /usr/sbin/sendmail -oi -f ${sender} 
${recipient}

I guess that is where the user "nobody" originates from. How do I get it to 
call spamc as the user the mail is for?
-- 
Sincerely,
Michael