Recurrr - Send recurring emails
RGB Technologies - Email Scheduler - Email Scheduler provides an intuitive email scheduling solution, integrated within the existing Salesforce account.
Right Inbox - Schedule Emails in Gmail To Be Sent Later
EmailPlayground - EmailPlayground provides solution to schedule and send emails with ease.
SendRecurring - SendRecurring allows you to setup, send, and maintain scheduled email reminders.
The Top Inbox - The Top Inbox for Gmail helps to schedule emails to be sent later, set reminders in the inbox and follow up with email sequences.