In order to have your users be able to send mail through the Plesk server, port 587 needs to be open. To do this, you must log in as the admin, and go to Server -> Mail and check Enable message submission. If that still does not work:
- No firewall rules are blocking the port 587
- Make sure port 587 is open and listening
# netstat -t -p | grep 587
- locate the file: submission_psa or submission.psa, the file should be located in the /etc/xinet.d/ directory.
By checking Enable message submission, Plesk renames submission_psa to submission.psa
If the file does not exist at all then you will need to create it. (See attached file)
Also, you will need to add submission to /etc/xinetd.conf.