Our app works fine on development. We are setting up Amazon SES, and it works perfectly on local.
We use rails credentials, and have a master.key, a development.key, and a production.key.
But as soon as we push the master key live, the app crashes.
Why is this happening?
It turns out that the the rails master key is irrelevant if you are using keys on an environmental basis, i.e. development.key and production.key.
Therefore, when launching live on Heroku, you do not want to use the master.key. You want to use the production.key.
This is what finally solved the heroku ses issue we were having.
How to properly set that production.key?
First: it is still called RAILS_MASTER_KEY on Heroku.
Second: you can either post the production.key to Heroku via the console:
heroku config:set RAILS_MASTER_KEY=`cat config/credentials/production.key`
Or you can set it manually on heroku in the 'settings' section by clicking on 'Environmental Variables'.