Postfix and Procmail

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@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
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