In /etc/postfix/main.cf, add:

smtpd_sasl_type = dovecot
smtpd_sasl_path = private/auth

and add the following subsection to the “auth default” section of /etc/dovecot/dovecot.conf:

socket listen {
client {
# Assuming the default Postfix $queue_directory setting
path = /var/spool/postfix/private/auth
mode = 0660
# Assuming the default Postfix user and group
user = postfix
group = postfix
}
}

Then you should be able to send email using the full realm as the username when authenticating (i.e. user@domain).

Feb 11 17:02:28 ping postfix/pipe[13448]: A20625E5ED: to=, relay=maildrop, delay=941, delays=941/0.01/0/0.02, dsn=4.3.0, status=deferred (te
mporary failure. Command output: ERR: authdaemon: s_connect() failed: No such file or directory .mailfilter(5): Syntax error after } )

#/etc/init.d/postfix restart
Stopping Postfix Mail Transport Agent: postfixpostfix: fatal: /etc/postfix/main.cf, line 40: missing ‘=’ after attribute name: “smtp inet n – - – 2 smtpd”
failed!
#

In your post you have the entry over three separate lines, but I figure it is really only two lines; am I correct?
smtp inet n – - – 2 smtpd
-o content_filter=spamassassin

spamassassin unix – n n – - pipe
user=spamd argv=/usr/bin/spamc -f -e /usr/sbin/sendmail -oi -f ${sender} ${recipient}