\documentclass[a4paper]{scrlttr2}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[ngerman]{babel}
\usepackage{url}
\usepackage{xcolor}
\newcommand{\todo}[1]{\textbf{\textcolor{red}{(TODO: #1)}}}
\usepackage[default,regular,black]{sourceserifpro}
\usepackage[T1]{fontenc}
\KOMAoptions{foldmarks=off,frommobilephone, fromemail}
\setkomavar{fromname}{Tim Buchwaldt}
\setkomavar{fromaddress}{Lukasstr. 16a\\50823 Köln}
\setkomavar{frommobilephone}[Mobil: ]{+49 151 16914796}
\setkomavar{fromemail}{tim@chaos.koeln}
\setkomavar*{enclseparator}{Anlage}

\begin{document}
\begin{letter}{\todo{Add recipient address}}

\setkomavar{subject}{\todo{Add Title}}
\opening{Sehr geehrte Damen und Herren,}

\todo{Add text}

\closing{Mit freundlichem Gruß,}
%\encl{Wichtiges Schreiben im Anhang}
\end{letter}
\end{document}