I understand that there are differences between transactional emails and newsletters, but most often the underlying email/smtp architecture works with both (in essence Mailgun is also an SMTP service, very similar to Amazon SMTP service but closer to SES in terms of functionalities).
I just probably see things a bit differently in the sense that I associate newsletter distribution more with an integration with Mailchimp (or TinyLetter, MailJet, SendinBlue, MailerLite or any other newsletter platform that also provides email templating services, i.e. Substack even) rather than Mandrill or Mailgun or AWS SES/SMTP and other similar services.
This is the reason why to me it would make more sense to implement a standard integration with an email newsletter service (MailChimp or other) rather than just with an SMTP provider.
This also comes from the impression that you guys seem to prefer implementing most non core features (comments or contact forms, to name a few that the community considers important for a CMS like Ghost) through integrations with third party providers that do those things better.
But then all this is just about how different people see different features as being core or not.