* [Mailpit v1.29.3](https://github.com/axllent/mailpit) – Small, fast, low memory email testing tool with an API for developers.

* [smtpmock v2.5.2](https://github.com/mocktools/go-smtp-mock) – Go-based configurable multithreaded SMTP mock server for testing, supporting RFC 2821/5321 and customizable command behavior.