The main problem with fa fa email is that it can be easily spoofed. This means that someone could create an email that looks like it is from a trusted source, but actually contains malicious content. This could allow someone to steal your information or even attack your computer.
<i class="fa fa-envelope"></i>
This code line is creating an envelope icon using the Font Awesome library.
insert fa fa class
An insert fa fa class is used to create a new style for an element. This class is typically used to create a new font-family for an element, but it can also be used to create a new color or other style.