We have several solutions to automatically send emails emitted by a Drupal 8 project in HTML format. Without being able to quote them all, we can use SwiftMailer, MimeMail to send mails from the server itself, or Mailjet API, MailGun, etc. to send emails from a third-party platform. In a few clicks, we can then emit different emails, whether they are transactional (account creation, order creation, subscription, etc.) or business (Newsletters, Activity Log, What you missed, etc. .), in HTML format. It will then remain to implement one (or more) responsive email template that will be correctly read and interpreted on most mail software. And it's probably the most important part. There remains a special case: that of all the emails emitted according to the different events related to the life of a user account (Creation, Waiting for approval, Deletion, Welcome Mail, etc.). Let's see how to simply send this emails in a HTML format too.