New Postfix + Spamassassin setup bounces mail back with "service unavailable" error.

Issue

Mail bounces back to sender with "<MAILER-DAEMON@>: service unavailable" error after configuring postfix with spamassassin as a delivery agent in master.cf:

smtp      inet  n       -       n       -       -       smtpd
         -o content_filter=spamassassin 
pickup    fifo  n       -       n       60      1       pickup
cleanup   unix  n       -       n       -       0       cleanup
<skip>
spamassassin unix  -       n       n       -       -       pipe
    user=spamx argv=/usr/bin/spamc -u spamx -e  /usr/sbin/sendmail -oi  -f ${sender} ${recipient}