Description
Send mail to a remote SMTP server (e.g. a transactional email provider like Postmark).
You should put your authentication details in settings.php, rather than files you commit to the repo, for security.
To get this to work - it was sending tests but nothing else - it seemed I needed to switch to a commit on the dev branch:
"drupal/smtp": "dev-1.x#3d354b3911409c9b11716bf2104498fb431eea72"
(note to self: see SM notes)