Mailtrack is an email tracking tool. It tells users if their emails have been opened or not. When the receiver reads an email, the message is marked with a check, and the system sends a read notification to the Mailtrack user.
Mailtrack did something very similar to what Hotmail had done many years before.
The Growth Hack
All Mailtrack users with a free account have a signature incorporated into their email messages. In this manner, their users share the service via email with anyone they send it to.
This strategy helped them reach millions of active users with very little investment.
Yes, it’s been done before, but that’s perfectly fine to copy (or take inspiration) a method that works and use it to grow your product.