It could be nice to configure a domain as a sandbox and having custom rules for outgoing emails.
When you have a production and other environments, you only want your production environment being able to send emails to your customers. Other environments use a custom sandbox that filter emails and only do logging. In that custom sandbox, you can configure a whitelist domain and emails not part of the whitelist domains and only go in the logs.
We have this situation actually where a system use Mailgun for transactionnal emails. Some of those emails are invoices or other communication sent to customer and emails for our users (password reset, etc). So using a sandbox with white list, we prevent test emails to go to our customers and keep our users (part of the whitelist domain) emails go thru.