social/mass_mailing_resend/views/mass_mailing_views.xml

<?xml version="1.0" encoding="utf-8"?>
<odoo>

    <record model="ir.ui.view" id="view_mail_mass_mailing_form">
        <field name="model">mail.mass_mailing</field>
        <field name="inherit_id" ref="mass_mailing.view_mail_mass_mailing_form"/>
        <field name="arch" type="xml">
            <field name="state" position="before">
                <button string="Resend"
                        states="done"
                        name="button_draft"
                        type="object"
                />
            </field>
            <header position="after">
                <div class="oe_form_box_info bg-warning oe_text_center" attrs="{'invisible': ['|', ('state', '!=', 'draft'), ('sent', '=', 0)]}">
                    <p>
                        <i class="fa fa-info-circle"/> New sending will be done only to not sent/new recipients. If you want to resend again the mass mailing to already sent recipients, click on <b>Emails Sent</b> smart-button for removing the existing record(s).
                    </p>
                </div>
            </header>
            <div name="button_box" position="attributes">
                <attribute name="attrs">{'invisible': [('sent', '=', 0)]}</attribute>
            </div>
        </field>
    </record>

</odoo>