social/mail_full_expand/views/mail_full_expand.xml

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

<!-- Copyright 2014-2015 Grupo ESOC <http://www.grupoesoc.es>
     License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl.html). -->

<odoo>
    <record id="mail_message_view_form" model="ir.ui.view">
        <field name="name">mail.message.full_expand</field>
        <field name="model">mail.message</field>
        <field name="priority">30</field>
        <field name="arch" type="xml">
            <form string="Message">
                <group>
                    <field name="author_id" readonly="1"/>
                    <field name="email_from" readonly="1"/>
                    <field name="date" readonly="1"/>
                    <field name="partner_ids"
                           widget="many2many_tags"
                           readonly="1"/>
                </group>
                <h1><field name="subject" readonly="1"/></h1>
                <field name="body" readonly="1"/>
                <field name="attachment_ids"
                       widget="many2many_binary"
                       readonly="1"/>
                <footer>
                    <button string="Close" class="btn-secondary" special="cancel"/>
                </footer>
            </form>
        </field>
    </record>

    <record id="mail_message_action" model="ir.actions.act_window">
        <field name="name">Read Full Email</field>
        <field name="res_model">mail.message</field>
        <field name="src_model">mail.message</field>
        <field name="type">ir.actions.act_window</field>
        <field name="view_id" ref="mail_message_view_form"/>
        <field name="view_type">form</field>
        <field name="view_mode">form</field>
        <field name="target">new</field>
    </record>
</odoo>