WhatsApp's newest update now allows users to know not only if his message was successfully sent but also if it was read. Described as the "read receipt" feature, the notification appears as blue double check marks placed at the end of the sent message.

In the past, there were only two types of check marks that users saw after sending a message. One check mark indicated that his message was successfully sent. Two check marks indicated that the message was successfully delivered to the intended recipient. Both types are in shades of gray.

With the new feature, the two check marks change their color to blue, which means the message has already been read.

WhatsApp's FAQ page explains that the new feature is also applicable in broadcast messages and group chat. The two gray check marks will turn blue once all the participants or recipients in the group have read the message. It has been pushed over-the-air to both Android and iOS. This means that the app becomes automatically updated, eliminating the need to install it from the App Store in order for it to start working.

"If you see two blue check marks next to your sent message, then the recipient has read your message. In a group chat or broadcast message, the check marks will turn blue when every participant has read your message," says the company.

WhatsApp also explains some possible reasons why the two gray check marks remain gray.

"If you do not see two blue check marks next to your sent message: 1. The recipient may not have opened your conversation 2. The recipient may be using an outdated version of WhatsApp 3. You or the recipient may be experiencing connection issues," says WhatsApp.

People have been waiting for WhatsApp to be updated with its highly anticipated voice calling feature after the company was acquired by Facebook in February under a financial deal that cost $19 billion. However, WhatsApp CEO Jan Koum said that they have no choice but to delay it until the first quarter of 2015.

The service is yet to overcome certain technical challenges, which include slow network connectivity plaguing the emerging markets and lack of access to some devices.

According to Koum, WhatsApp now has 600 million monthly active users worldwide.

Now serving 600,000,000 monthly active users. Yes, active and registered are very different types of numbers...

— jan koum (@jankoum) August 25, 2014

This number is an increase of more than 100 million users from the monthly active users in April 2014, which was recorded to have reached 500 million.