Hey Damien,

You need to make Send by email button invisible or replace existing button and add a custom new button. 

Inherit sale.order class and form view to python and xml respectively.

Python:

    @api.multi
    def action_quotation_sent_without_mail(self):
        return self.write({'state': 'sent'})

XML:

    <xpath expr="//button[@name='action_quotation_send']" position="after">
            <button name="action_quotation_sent_without_mail" string="Quotation Sent" type="object" states="draft" class="btn-primary"/>
    </xpath>