Qmail Service does not start in Plesk :

/etc/xinetd.d/smtp_psa file could be edited manually:

Hence replace smtp_psa file with default settings.
Try restart Qmail.

Restarted services:
root@uhweb28187:/# /etc/init.d/xinetd restart Stopping internet superserver: xinetd.
Starting internet superserver: xinetd.
root@uhweb28187:/# /etc/init.d/qmail restart Starting qmail: done

Done,...