ActionMailer not delivering mail from one particular server

We have a client who insisted on having their Rails app set up with Oracle DB on a Red Hat Enterprise Linux server behind their VPN. This is not a setup we are very familiar with as we generally run DigitalOcean droplets with Ubuntu and use MySQL/PostgreSQL. Needless to say, this has caused a lot of problems, including the latest mystery that we’ve yet to solve. We have this app running on an Ubuntu staging server where everything works fine, but the copy running on the RHEL server will not deliver emails through ActionMailer.

We use Postmark as our delivery service, and so far we have ruled out: Postmark, the VPN, firewalls, the Postmark gem, and the Mail gem. I’ve written several test scripts to test each layer, and the failure is at the ActionMailer level. Here are the tests:

require ‘postmark’

client = Postmark::ApiClient.new(ENV[‘API_TOKEN’])

client.deliver(