Nodemailer

Nodemailer is the most widely-used email sending library for Node.js, originally created by Andris Reinman in 2011 and maintained by him and the open-source community. It has become the de facto standard for programmatic email sending in the Node.js ecosystem, powering email functionality in countless applications ranging from transactional email systems to marketing automation platforms. Nodemailer's architecture consists of three main components: the Transporter (the core sending engine), the Message Object (defining email content and metadata), and Transports (pluggable delivery backends). The library supports a wide range of transport mechanisms: SMTP (the most common, supporting TLS, STARTTLS, authentication via plain/login/oauth2, connection pooling, and custom DKIM signing), Sendmail (invoking the local sendmail binary), Amazon SES (via the nodemailer-ses-transport plugin), Stream (for testing or custom delivery), and JSON (for logging or third-party APIs). Key features include support for HTML and plain text dual-format emails, file attachments (from files, streams, strings, or URLs), inline images via Content-ID, embedded images, calendar events (iCal attachments), custom headers, message priority levels, delivery status notifications (DSN), Unicode support for internationalized email addresses and subject lines, and comprehensive error handling with detailed bounce information. Nodemailer also provides powerful email template support through Handlebars, Pug, or EJS integrations. With over 17,000 GitHub stars and over 3 million weekly npm downloads, Nodemailer is trusted by enterprises and startups alike. The library is licensed under the MIT license.