Simplifies the interface for nodemailer and returns a promise. Also includes TypeScript declaration file for type checking. Credentials for the email service can be passed in the MailPromise class constructor or through the environment variables. Setting the credentials from the environment variables is useful for when you want to use mail credentials in multiple projects but not risk having them exposed. To set the credentials from the environment variables you need to create one for the service, the username, and the password. The file you edit will vary depending on your system. I set mine on Linux from /etc/environment.