OffHours for Outlook VS Batched Inbox

RGB Technologies - Email Scheduler - Email Scheduler provides an intuitive email scheduling solution, integrated within the existing Salesforce account.

SendLater - Email scheduler for Mac Mail.

EmailPlayground - EmailPlayground provides solution to schedule and send emails with ease.

Right Inbox - Schedule Emails in Gmail To Be Sent Later

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.

SndLatr - SndLatr is a Gmail plugin that makes it easy to schedule emails to be sent automatically at a later time.