In ir.ui.view, I have found sale.order.form :

 <button name="action_quotation_send" string="Send by Email" type="object" states="sent,progress,manual" groups="base.group_user"/>

In the file addons/sale/sale.py, we have the definition of action_quotation_send :

    def action_quotation_send(self, cr, uid, ids, context=None):
        '''
        This function opens a window to compose an email, with the edi sale template message loaded by default
        '''
        assert len(ids) == 1, 'This option should only be used for a single id at a time.'
        ir_model_data = self.pool.get('ir.model.data')
        try:
            template_id = ir_model_data.get_object_reference(cr, uid, 'sale', 'email_template_edi_sale')[1]
        except ValueError:
            template_id = False
        try:
            compose_form_id = ir_model_data.get_object_reference(cr, uid, 'mail', 'email_compose_message_wizard_form')[1]
        except ValueError:
            compose_form_id = False
        ctx = dict()
        ctx.update({
            'default_model': 'sale.order',
            'default_res_id': ids[0],
            'default_use_template': bool(template_id),
            'default_template_id': template_id,
            'default_composition_mode': 'comment',
            'mark_so_as_sent': True
        })
        return {
            'type': 'ir.actions.act_window',
            'view_type': 'form',
            'view_mode': 'form',
            'res_model': 'mail.compose.message',
            'views': [(compose_form_id, 'form')],
            'view_id': compose_form_id,
            'target': 'new',
            'context': ctx,
        }