mail-addons/mail_delete_sent_by_footer.py

from openerp.osv import osv
from openerp import tools, SUPERUSER_ID

class mail_notification(osv.Model):
    _inherit = 'mail.notification'

    def get_signature_footer(self, cr, uid, user_id, res_model=None, res_id=None, context=None, user_signature=True):
        footer = ""
        if not user_id:
            return footer

        # add user signature
        user = self.pool.get("res.users").browse(cr, SUPERUSER_ID, [user_id], context=context)[0]
        if user_signature:
            if user.signature:
                signature = user.signature
            else:
                signature = "--<br />%s" % user.name
            footer = tools.append_content_to_html(footer, signature, plaintext=False)

        return footer