The email templates allow you to customise the messages that go out to your customers when actions occur inside WHMCS. Every email that goes out to a customer can be customised here.
To edit an email template, go to Setup > Email Templates and click the edit icon next to the template you want to change.
You can also toggle plain text vs. HTML email sending on a per email basis from the editor page.
Creating Custom Templates
You can create your own email templates to serve as custom welcome emails for products or messages you send manually but frequently
Now the email will be listed in the appropriate place for sending. Please refer to the Messages/Emails page.