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.
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.
Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.
FollowUpThen - The simplest way to schedule an email reminder
InstAddr - Disposable email address application.