SMTP / PHPMailer module in core

Following this discussion on Gitter :

@herbdool We should have an easy way to configure email for sending HTML and via SMTP. Should be much easier than the Drupal 7 setup with Mailsystem

@opi I use https://github.com/makinacorpus/drupal-netsmtp for basic SMTP support in D7. No UI. btw smtp support is good not only for HTML mails, but also to plug backdrop with services like mailjet, mailchimp... would a core support be an improvment for the 80%?

@quicksketch I believe SMTP support is a requirement for all systems that send email (including Backdrop) these days. Sending email is just so hard. It rarely works (getting into actual inboxes) without an external service. So in my opinion, yes I think it's a core candidate.

Describe your issue or idea

Administration interface should provide an easy way to configure an external SMTP server to send every email from Backdrop.