Mailtrack.io - The best email tracking solution I've found. Free
Roundcube - Web-based IMAP email client
Saleshandy - Saleshandy is a cold email outreach platform that enables marketing and sales teams to scale their email outreach operations seamlessly.
HEY - Email at its best, new from Basecamp.
Gmelius - Gmelius is the first collaboration platform designed for G Suite and your company's daily tools.
Mailo - Mailo is an email client where you can send and receive emails to and from anyone with an email address.