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.
If No Reply - Simple email automation that works for Gmail
Gmelius - Gmelius is the first collaboration platform designed for G Suite and your company's daily tools.
Assistant.to - GoToMeeting. Put your GoToMeeting link in the Assistant. to "Location" field. Once the recipient selects a time, you will both receive an invite with a unique GoToMeeting link.
Boomerang for Outlook - Boomerang for Outlook offers email productivity tools to track, schedule and send mails at the right time.
SendLater - Email scheduler for Mac Mail.