social/mass_mailing_partner/views/mailing_trace_view.xml

<?xml version="1.0" encoding="utf-8" ?>
<!-- Copyright 2016 Antonio Espinosa - <antonio.espinosa@tecnativa.com>
     Copyright 2020 Tecnativa - Manuel Calero
     License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl.html). -->
<odoo>

<record model="ir.ui.view" id="mailing_trace_view_form">
    <field name="name">Add partner field</field>
    <field name="model">mailing.trace</field>
    <field name="inherit_id" ref="mass_mailing.mailing_trace_view_form" />
    <field name="arch" type="xml">
        <field name="message_id" position="after">
            <field name="partner_id" />
        </field>
    </field>
</record>

<record model="ir.ui.view" id="mailing_trace_view_tree">
    <field name="name">Add partner column</field>
    <field name="model">mailing.trace</field>
    <field name="inherit_id" ref="mass_mailing.mailing_trace_view_tree" />
    <field name="arch" type="xml">
        <field name="sent_datetime" position="before">
            <field name="partner_id" />
        </field>
    </field>
</record>

<record model="ir.ui.view" id="mailing_trace_view_search">
    <field name="name">Add partner search field and group by</field>
    <field name="model">mailing.trace</field>
    <field name="inherit_id" ref="mass_mailing.mailing_trace_view_search" />
    <field name="arch" type="xml">
        <field name="mass_mailing_id" position="after">
            <field name="partner_id" />
        </field>
        <filter name="group_reply_date" position="after">
            <filter
                    string="Partner"
                    name="group_partner"
                    context="{'group_by': 'partner_id'}"
                />
        </filter>
    </field>
</record>

</odoo>