Send emails from a Node.js app using SMTP, transactional email APIs, or multichannel notification services. Each method has its pros and cons: SMTP offers fine-grained control but is insecure and slower; transactional email APIs are easy to set up and scalable but rely on third-party providers; and multichannel notification services allow for easy integration with multiple channels using one uniform API. Nodemailer is a popular Node.js module for sending emails using SMTP, while SendGrid and Mailgun offer transactional email APIs. Courier provides a multichannel notification service that integrates well with various email providers and offers customizable templates and analytics.