Swaks is a featureful, flexible, scriptable, transaction-oriented SMTP test tool written and maintained by John Jetmore. It is free to use and licensed under the GNU GPLv2. Features include:
The official project page is https://jetmore.org/john/code/swaks/.
The latest version of Swaks is 20240103.0 (announcement), which can be downloaded as a package or a standalone script.
See the installation page for details on installing in multiple environments.
There is also a versions page which lists every released version of Swaks, complete with changelogs and download links.
The reference documentation from the latest release, which includes quick-start examples, is available as plain text and rendered. The documentation from each release is available from the versions page. There is also an Occasionally Asked Questions document.
Feedback and meaningful questions about how to use Swaks are welcome. However, since Swaks is only maintained by a single person as a hobby, there is no guarantee of a timely response.