CMail - CMail is a free Windows command line mailer with advanced message formatting options.
RingCentral - RingCentral is the leading provider of cloud-based communications and collaboration solutions for small business and enterprise companies
mailsend-go - mailsend-go is a command line tool to send mail via SMTP protocol, written with golang.
Dialpad - Switch is a cloud-based phone system built for Google Apps users.
sendEmail - sendEmail is a lightweight, command line SMTP email client.
Aircall - Aircall is a call center software of a new generation designed for fast growing companies. Setup instantly and integrates to your CRMs