rsc.io/mailgun
This repo holds basic utilities for interacting with the Mailgun email service.
rsc.io/mailgun/cmd/mailgun-mail is a drop-in replacement for the mail-sending mode of BSD mailx.
rsc.io/mailgun/cmd/mailgun-sendmail is a drop-in replacement for the mail-sending mode of the sendmail daemon.
The idea behind both these programs is that you can install them in place of the usual mail and sendmail programs, and then programs can still send mail from your local system, without having to configure and run a full-blown mail system.