Adding envelope symbol for corresponding authors in LaTeX documents

Use the package ifsym

\usepackage{ifsym}

and then you can use the command

\Letter

which works as regular text, not as math. This means you have to make sure you are using regular text where you put the envelope symbol to appear

\author{Yo Mero$^{\textrm{\Letter}}$}

In other words, if you use

\author{Yo Mero$^\Letter$}

a different symbol will appear in your document.

To make it easier to write, you can create a new command like

\newcommand{\corrAuthor}{$^{\textrm{\Letter}}$}