Modify the process_email_queue adding limit argument at the search method (here limit=400)

def process_email_queue(self, cr, uid, ids=None, context=None): """Send immediately queued messages, committing after each message is sent - this is not transactional and should not be called during another transaction!

       :param list ids: optional list of emails ids to send. If passed
                        no search is performed, and these ids are used
                        instead.
       :param dict context: if a 'filters' key is present in context,
                            this value will be used as an additional
                            filter to further restrict the outgoing
                            messages to send (by default all 'outgoing'
                            messages are sent).
    """
    if context is None:
        context = {}
    if not ids:
        filters = ['&', ('state', '=', 'outgoing'), ('type', '=', 'email')]
        if 'filters' in context:
            filters.extend(context['filters'])
        ids = self.search(cr, uid, filters, context=context, limit=400)
    res = None
    try:
        # Force auto-commit - this is meant to be called by
        # the scheduler, and we can't allow rolling back the status
        # of previously sent emails!
        res = self.send(cr, uid, ids, auto_commit=True, context=context)
    except Exception:
        _logger.exception("Failed processing mail queue")
    return res