I need to send out a mail when a PoS session is closed:

@api.multi
@api.constrains('state')
def _session_resume(self):
    if self.state == 'closed':
        print 'closed'
        mail = self.env['mail.mail']

        mail_data = {'subject': 'closing session : ' + datetime.now().strftime("%d-%m-%Y"), 
                     'body_html': 'test', 'email_from': '******@gmail.com',
                     'email_to': '******@gmail.com'}

        mail_out = mail.create(mail_data)
        mail.send(mail_out)

I can reach without issues the api.constrains statement, but no mail is coming out, at least not immediately. 
Forcing an email cron job on automation/ scheduled actions  I can clearly see an email log "successfully sent" and my mail is delivered:

2019-01-28 19:37:58,179 1164 INFO db02 odoo.addons.mail.models.mail_mail: Mail with ID 6 and
 Message-Id u'<1548704145.344115972518921.633285188501212-openerp-private@HP-Note>' successfully sent

Then how can I force an immediate email from my python code without edit the stock time (1 hour) in Email Queue Manager?