Gmelius - Gmelius is the first collaboration platform designed for G Suite and your company's daily tools.
Saleshandy - Saleshandy is a cold email outreach platform that enables marketing and sales teams to scale their email outreach operations seamlessly.
Mailtrack.io - The best email tracking solution I've found. Free
Followup.cc - FollowUp for email is a lightweight productivity suite that keeps your conversations and tasks from slipping through the cracks. Set perfectly timed reminders that arrive at the top of your inbox, right when you need them.
Mailbutler - Mailbutler adds lacking functionality to email clients and increases your productivity
MailTracker by J David Baker - Package for Laravel to inject tracking code into outgoing emails. - GitHub - jdavidbakr/mail-tracker: Package for Laravel to inject tracking code into outgoing emails.