Clean Email - Clean Email is an online service that empowers you to take control of your mailbox.
SendRecurring - SendRecurring allows you to setup, send, and maintain scheduled email reminders.
Hiver - Manage Customer Support, Sales and Projects right from your Gmail inbox. Collaborate with Shared Labels, Shared Contacts, email notes, email reminders and email templates
Recurrr - Send recurring emails
MailClark - The Slack bot for external communications
FollowUpThen - The simplest way to schedule an email reminder