By default, the submission port is TCP-587. To change to another value, edit the following file :
/var/qmail/supervise/submission/run
And simply change the port number just before the last line :
/usr/bin/tcpserver -v -R -H -l $HOSTNAME -x $TCP_CDB -c "$MAXSMTPD" \ -u "$QMAILDUID" -g "$NOFILESGID" 0 587 \ $SMTPD $VCHKPW /bin/true 2>&1
Of course, you need to restart qmail to make it effective.