CMail is a command line e-mail sending tool. It was originally developed to assist with testing mail server and content analysis software. As a result, CMail provides many more message formatting and SMTP session control options than similar software.
MailChimp - MailChimp is the best way to design, send, and share email newsletters.
blat - Blat is a small, efficent SMTP command line mailer for Windows.
Constant Contact - Constant Contact offers email marketing, social media marketing, online survey, event marketing, digital storefronts, and local deals tools.
mailsend-go - mailsend-go is a command line tool to send mail via SMTP protocol, written with golang.
Campaign Monitor - Email marketing software built for designers and their clients to run successful email campaigns.
sendEmail - sendEmail is a lightweight, command line SMTP email client.