AwesomeFont Email Icon showing 'S' rather than envelope on different computers

On the button on the home page where it says “Signup For Newsletter”, some computers show the envelope icon, others show an ‘S’ letter. My PC running Windows 10 shows the envelope in Chrome, while my Laptop and my client’s Laptop, running Windows 10 and Windows 7, show the ‘S’ icon and not the envelope. This did not start occurring until Upfront was updated.