\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary{external}
\tikzexternalize[only named]
\tikzset{
    export as png/.style={
        external/system call/.add={}{
            && convert -density #1 -transparent white "\image.pdf" "\image.png"
        },
    },
    export as png/.default={200},
}
\begin{document}
    \tikzset{export as png=500}
    \tikzsetnextfilename{email-png}
    \begin{tikzpicture}
        \node {fsinf@uni-frankfurt.de};
    \end{tikzpicture}
\end{document}