\documentclass[AlbrechtMehl%
,ngerman%
%,english%
%,french%
,headsepline]{scrlttr2}

\AMehlOptions{Tel,Int}

\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}

\textheight28cm
\nofiles

\setkomavar{date}{2018-11-26}

\begin{document}
\ttfamily

\begin{letter}{Herrn\\
               Fritz Muster}

\setkomavar{subject}{\\
                     \noindent{}}
                     
\opening{Sehr geehrter Herr Muster,}

HALLO

\closing{Hochachtend}
\end{letter}
\end{document}