The new deployment-mx routes emails from the beta-cluster - and still have the $domain part of Return-Path header set to 'wikimedia.org'. This will cause for bounces to come back all the way till polonium - which is not the intended path. Probably, beta should have something like wmflabs.org set as the $domain part so that bounce emails will come back to deployment-mx.
The current return path for an email sent from http://deployment.wikimedia.beta.wmflabs.org/ is :
Return-Path: <wiki-deploymentwiki-pg-niuja1-g/Qo8rG/uI9DaDAg@wikimedia.org>
After adding DNS records for deployment-mx. This should become something like :
Return-Path: <wiki-deploymentwiki-pg-niuja1-g/Qo8rG/uI9DaDAg@wmflabs.org