Recs.
Updated
Specs
Pros
Pro Can use feature w/ just about any Gmail client
Since this feature works based on adding a string to the end of the subject line and is monitored from the cloud, you can use and change the send later time/date in just about any Gmail client where you can access drafts and change the subject line.
Cons
Con Syntax for setting up is a bit technical
You have to add things like "@2017-07-01 04:00 repeat every month" at the end of the subject in your drafts folder. There is no convenient/intuitive interface or natural language to make this easier.
Take a look at the documentation to see if this is for you.
Con Does not uninstall cleanly
Many users are getting emails from Google Scripts with error logs every day because of Mail Conductor. Instructions to uninstall are to install the add-on again, and then to click the uninstall button at the bottom of the screen where a few options/settings are.
Con Difficult to send an email in a different time zone
This service is limited to the selected time zone when installing the service. To change the user's time zone, one must update the setting by reinstalling the service. Users of this service must offset the time difference between user and destination and set the (offset calculated) time in the user's time zone.