Thoughts that are my own.

Enable Submission Port on a Plesk Server

First, copy the smtp_psa file.

cp /etc/xinetd.d/smtp_psa /etc/xinetd.d/smtp_additional

Edit the newly copied file and replace the first line. Replace ‘service smtp’ with ‘service submission’

vi /etc/xinetd.d/smtp_additional

Restart xinetd.d

service xinetd restart

Check that port 587 is listening on the server

netstat -ltnp