Can't connect to Postfix smtp server from mupx docker

I’m using mupx to install a meteor app on AWS. I need to send lots of emails and would like to run my own smtp server. I’ve followed

When the app tries to send an email I get the error:

Exception in setTimeout callback: Error: connect ECONNREFUSED
    at Object.Future.wait (/bundle/bundle/programs/server/node_modules/fibers/future.js:449:15)
    at smtpSend (packages/email/email.js:89:1)

Looking here I’ve also added to mynetworks.

The swacks test works fine.

Any ideas how to get Postfix to accept a connection from the docker meteor app?