there is a template already in openerp to do that , go to setting-->technical --> email --> templates , you will find template "on task create" which send automatic email to assigned user email

Thanks for your answer.

However everything sounds ok the only problem was my email sending scheduler.

The Scheduled Action "Email Queue Manager" will send it. It runs every hour, by default.

At: Settings -> Technical -> Automation -> Scheduled Actions -> Email Queue Manager

I set it to 5 minutes and I'm testing now.