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.
Outlook4Gmail - Outlook4Gmail provides easy synchronization of your Outlook and Google accounts
Gmelius - Gmelius is the first collaboration platform designed for G Suite and your company's daily tools.
Boomerang for Outlook - Boomerang for Outlook offers email productivity tools to track, schedule and send mails at the right time.
BlueMail - BlueMail is an email management app for Android, iOS, and Windows.
FollowUpThen - The simplest way to schedule an email reminder