Right Inbox - Schedule Emails in Gmail To Be Sent Later

Nylas Mail - The Nylas Cloud API powers your application with email, calendar & contacts features. Built-in features for better email, calendar, and contact management.

Recurrr - Send recurring emails

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

FollowUpThen - The simplest way to schedule an email reminder

Clean Email - Clean Email is an online service that empowers you to take control of your mailbox.