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:

* SMTP extensions including TLS, authentication, pipelining, and XCLIENT
* Protocols including SMTP, ESMTP, and LMTP
* Transports including UNIX-domain sockets, internet-domain sockets (IPv4 and IPv6), and pipes to spawned processes
* Completely scriptable configuration, with option specification via environment variables, configuration files, and command line