* [Haraka v3.2.0](https://github.com/haraka/Haraka) – Highly scalable email server with a modular plugin architecture.

* [MailTime 4.1.0](https://github.com/veliovgroup/mail-time) – Email queuing system that extends Nodemailer with multi-SMTP rotation, storage-backed retries, and horizontal scaling support.

* [smtp-server v3.18.5](https://github.com/nodemailer/smtp-server) – Node.js module for creating SMTP and LMTP server instances dynamically.

* [Maddy Mail Server v0.9.5](https://github.com/foxcpp/maddy) – Composable all-in-one mail server implementing SMTP (MTA/MX), IMAP storage, and security protocols such as DKIM, SPF, DMARC, and DANE.

* [Mokapi v0.43.0](https://github.com/marle3003/mokapi) – API mocking tool using OpenAPI and AsyncAPI to simulate HTTP, Kafka, LDAP, and SMTP with JavaScript-driven behaviors.