On plesk 8 and greater in Mail Server Settings turn on “Enable Message Submission”

plesk enable smtp port 587

If your clients still can’t send through port 587 do the following.

Enable Port 587 on Plesk when ISP’s block outgoing mail

You should create the file or add the record manually “submission_psa”

/etc/xinetd.d/submission_psa

contents:

service submission
{
socket_type = stream
protocol = tcp
wait = no
disable = no
user = qmaild
instances = UNLIMITED
env = SUBMISSION=1 SMTPAUTH=1
server = /var/qmail/bin/tcp-env
server_args = /var/qmail/bin/qmail-smtpd /var/qmail/bin/smtp_auth /var/qmail/bin/true /var/qmail/bin/cmd5checkpw /var/qmail/bin/true
}

restart xinetd

/etc/rc.d/init.d/xinetd restart