The api_user & api_pass could be extracted into a `dotmailer/credentials.rb`

```ruby
module Dotmailer
  def self.api_user
    ENV['DOTMAILER_USER'] || raise(MissingCredentials, 'DOTMAILER_USER is not set in the environment')
  end

  def self.api_pass
    ENV['DOTMAILER_PASS'] || raise(MissingCredentials, 'DOTMAILER_PASS is not set in the environment')
  end
end
```