social/mail_check_mailbox_size/views/fetchmail_view.xml

<?xml version="1.0"?>
<odoo>

    <record id="view_email_server_form_check_size" model="ir.ui.view">
        <field name="name">view_email_server_form_check_size</field>
        <field name="model">fetchmail.server</field>
        <field name="inherit_id" ref="fetchmail.view_email_server_form"/>
        <field name="arch" type="xml">
            <xpath expr="//notebook/page[2]/group[1]" position="after">
                <group string="Mailbox size checking" col="4" attrs="{'invisible': [('type', '!=', 'imap')]}">
                    <field name="check_mailbox_size"/>
                    <field name="mailbox_size_partner_ids" widget="many2many_tags"
                           options="{'no_create': True}"/>
                </group>
            </xpath>
        </field>
    </record>
</odoo>