I am able to send plaintext emails with the Simplenews module. I have installed the Mimemail module; I configured the mail system to use Mime Mail Mailer in DEFAULT MAIL SYSTEM. After sending the emails to queue, when I run the CRON job, Drupal displays an error message:
Unable to send email. Contact the site administrator if the problem persists.
Has anyone used Simplenews to send HTML emails in Drupal 8.1.1? If so, can you please advise how to do this?