Postfix and Procmail

[Nathan Aubrey]

What is the proper way to make use of spamassassin and procmail with the 
postfix mail agent? I keep getting errors saying 

               The mail system

<user at domain.com>: can't create user output file. Command output:
    procmail: Error while writing to "/var/spool/mail/user"


I have removed the offending file and sent messages and it works, but then 
begins doing this again.
I am using dovecot for pop/imap do they use different file formats or 
something?

nathan