`helo_hostname` is not fully realized

## Describe the bug

`Config.dns.helo_hostname`, commented with 'The hostname to use in HELO/EHLO when connecting to external SMTP servers'.

But in `app/lib/smtp_server/client.rb`:

```ruby
new_io.print("220 #{Postal::Config.postal.smtp_hostname} ESMTP Postal/#{client.trace_id}")
```

And a few other places.