I found this at base/res/res_partner.py:

def email_send(self, cr, uid, ids, email_from, subject, body, on_error=''):
        while len(ids):
            self.pool.get('ir.cron').create(cr, uid, {
                'name': 'Send Partner Emails',
                'user_id': uid,
                'model': 'res.partner',
                'function': '_email_send',
                'args': repr([ids[:16], email_from, subject, body, on_error])
            })
            ids = ids[16:]
        return True

The task is added to the cron and executed as soon as possible(i think with a delay of max. a minute) in background.