Issues sending HTML email with Simplenews module 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?
Check out the following modules, in their configuration it's easy to set email system and html format:


Mail System.
SMTP Authentication Support.


Select mime mail in the config of mail system. Also make sure that content text format and newsletter settings is full html and it should do it.