Ok, so I found out that what you have to do is not use the codes as mentioned on fontello:
U+E84D
U+E854
But rewrite these to:
\E84D
\E854
(so remove the "U+" and replace it with a "\")
Use them like so:
content: "\E84D";
EDIT:
So, on request, the complete CSS syntax you would use is:
.icon:before {
display: inline-block;
font-family: 'Entypo';
text-decoration: none;
speak: none;
}
.email:before {
content: "\E84D";
}
In combination with the following HTML:
<a href="#" class="icon email">Mail me</a>