Hello to everyone, here is the solution how to resolve this problem!
Problem i caused by unreliable SMTP connection between moodle and mail server,
so only one or few users from the course group receive mail message, all other mails were: Failed to send due to the following error: "SMTP connect()
The solution is to use local mail system at the same server with Moodle installation.
You only have to set relay mail server address in local mail system (exim4, postfix, at Linux servers), and to leave field SMTP host settings at Moodle empty (default).
Best Regards,
Darko Kozarski