Hi,
You can make a config file to register your email template.
Add your email template in .tpl format (it’s HTML code and use custom .tpl extension) in platform/plugins/[your-plugins]/resources/email-templates/[email-key].tpl
Load that config in your plugin service provider.
And register that template settings, following code will register email template in Settings → Email templates.
Then use can send mail using this template:
You can check our example in plugin Newsletter (platform/plugins/newsletter) or Contact (platform/plugins/contact)