To turn off SRS go to /etc/postfix/master.cf find line:
plesk_virtual unix - n n - - pipe flags=DORhu user=popuser:popuser argv=/usr/lib64/plesk-9.0/postfix-local -f ${sender} -d ${recipient} -p /var/qmail/mailnames
and add -s key to the end:
plesk_virtual unix - n n - - pipe flags=DORhu...