Allows leaving `smtp_settings[:domain]` unspecified. Explicitly setting it to `nil` (as when `OSEM_SMTP_DOMAIN` is unset) overrides the default value of `'localhost.localdomain'` and caused SMTP connections to fail with `EOFError`.
re: https://www.rubydoc.info/gems/rubocop/RuboCop/Cop/Style/FrozenStringLiteralComment
This reverts commit 1255088ff0. Because it broke our heroku deployment
1255088ff0