A client-side Send Later implementation does not work reliably since the device may not be powered on or have connectivity at the scheduled time, or the app may have been force-quit by the user.
As a result, most apps that offer Send Later / Email Scheduling opt for a a server-side implementation where the scheduled email is stored on a 3rd party server till it is sent at the scheduled time.
Obviously, this has privacy / security implications since not only do scheduled emails need to be stored on the 3rd party server, but said server also needs to be able to access the user’s email server via SMTP in order to send the emails.
While similar security / privacy considerations also apply to Push vs Fetch notifications, in our view the need to be informed of new incoming emails promptly and reliably justifies a server-side implementation in that case, while email scheduling, although nice to have, is not as critical.
