Boomerang for Gmail is a free chrome extension that will help you to schedule it to be sent automatically at the preferred time even if you’re not online. The service allows you to schedule emails to automatically send in the future, so you can write an email now, and the service will send it later.
You can easily add Boomerang in your chrome browser after the installation you can see an icon in your Gmail. Here you can set your mail in advance.
Boomerang also helps you to postpone incoming emails, by making them disappear from your inbox into a folder or label, then bringing them back to the top of the message list at the specified time. It helps you keep your inbox clean, without losing track of important messages. Boomerang is also the only reminder service that can alert you if you do not get a reply to a message.