Fixed:

class TheModel(models.Model):

    ...

    _inherit = ['mail.thread', 'mail.activity.mixin']

    ...

    @api.multi
    @api.returns('self', lambda value: value.id)
    def message_post(self, body='', subject=None, message_type='notification', **kwargs):
        kwargs['email_from'] = "REALMAIL <realmail@domain.com>"

        return super(TheModel, self).message_post(body, subject, message_type, **kwargs)