Email Scheduler for Gmail lets you schedule email messages inside Gmail with the help of a Google Spreadsheet. You can write your messages now and Gmail will send them later at your specified date and time. The scheduler can also send recurring emails that go out automatically on daily, weekly, monthly or yearly repetitive schedule.

You can use the Email Scheduler for Gmail to schedule birthday and anniversary greetings in advance, create recurring reminders (for events, employee appraisals, appointments), and more. You can write your emails in Gmail now, let the emails stay in the drafts folder of Gmail and delay the delivery. Gmail will automatically deliver your emails at the preferred date and time.

The premium version of Email Scheduler includes email tracking (know who has opened your email), you can schedule recurring messages and it also offers the option to create multiple copies of Gmail drafts. This is useful if you wish to send the same draft email to multiple recipients, separately. Get the premium edition from http://gum.co/GA09

Watch the video tutorial to know how email scheduling works inside Gmail with Google Sheets. 
Link: https://www.youtube.com/watch?v=MMTORprgUBI 

Visit the Gmail scheduler page to know more about scheduling and for answers to common questions.  Link: http://labnol.org?p=24867 

Upgrade to Premium from http://gum.co/GA09

Changelog
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v80 - Improved time trigger for scheduling emails, bug fixes
v59 - UI updates, performance enhancement, improved logging.
v48 - Bug Fixes, the scheduled date and time for a message should be at least 60 minutes prior. The sender's name is now preserved in the outgoing email messages.
v41 - Added option to restart email scheduler. The local timezone is now computed using the jsTimezoneDetect library. Logging improved. Misc bug fixes.

For support, email amit@labnol.org