It's an old post (apologies) but it was the virtually the same issue I have just experienced. After publishing a new version (with a trivial change) of a previously working "send email" module, all my new emails were stuck at "pending" on both non-production and production. Dev was working fine with the same code.
The resolution was provided by Outsystems Support. The module I use to send emails had to be republished (with no changes). Somehow a "behind the scenes" flag was missing and they don't know why (yet).
In Service Center go to factory, modules, <your module that sends the emails> and then hit "Redeploy published modules". All my emails stuck at pending were immediately sent much to my relief.