social/mass_mailing_resend/views/mailing_mailing_views.xml

<?xml version="1.0" encoding="utf-8" ?>
<odoo>
    <record id="view_mail_mass_mailing_form" model="ir.ui.view">
        <field name="model">mailing.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
                        >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>
    <record id="mailing_mailing_view_form_full_width" model="ir.ui.view">
        <field name="model">mailing.mailing</field>
        <field
            name="inherit_id"
            ref="mass_mailing.mailing_mailing_view_form_full_width"
        />
        <field name="arch" type="xml">
            <button name="action_view_traces_sent" position="attributes">
                <attribute
                    name="attrs"
                >{'invisible': [('state', 'in', ('draft','test')),('sent', '=', 0)]}</attribute>
            </button>
        </field>
    </record>
</odoo>