* [Inbucket v3.2.0](https://github.com/inbucket/inbucket) – Disposable webmail server providing built-in SMTP, POP3, REST and web interfaces without requiring a database.

* [Delta Chat Desktop v2.62.0](https://github.com/deltachat/deltachat-desktop) – Desktop application for secure messaging based on email protocols.

* [MailTime 5.3.0](https://github.com/veliovgroup/mail-time) – Email queuing system that extends Nodemailer with multi-SMTP rotation, storage-backed retries, and horizontal scaling support.

* [MailDev maildev@3.0.0](https://github.com/maildev/maildev) – SMTP server and web interface for capturing and viewing generated emails during development.

* [smtp-server v3.19.19](https://github.com/nodemailer/smtp-server) – Node.js module for creating SMTP and LMTP server instances dynamically.

* [Maddy Mail Server v0.9.6](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 v0.52.0](https://github.com/marle3003/mokapi) – API mocking tool using OpenAPI and AsyncAPI to simulate HTTP, Kafka, LDAP, and SMTP with JavaScript-driven behaviors.

* [MidiSmtpServer release/3.3.2](https://github.com/4commerce-technologies-AG/midi-smtp-server) – Highly customizable Ruby SMTP-Server and SMTP-Service library with built-in AUTH and SSL/STARTTLS, supporting 8BITMIME and SMTPUTF8 over IPv4 and IPv6.