\documentclass[letter]{scrlttr2}
\special{papersize=8.5in,11in}
]\usepackage[english]{babel}
\KOMAoptions{fromalign=left, fromlogo=false, addrfield=true,
backaddress=on, subject=titled, subject=left,foldmarks=on,
fromphone=on,fromemail=true}
\setkomavar{fromname}{Thomas Mustermann}
\setkomavar{signature}{Thomas Mustermann}
\setkomavar{fromaddress}{Some street 23\\Postcode City}
\setkomavar{fromemail}{t.mustermann@domain.com}
\setkomavar{fromphone}{00000000}
\setkomavar{subject}{Here goes the subject of the letter}
\begin{document}
\begin{letter}{Some firm\\some area\\some postcode\\some country}
\opening{Dear Ladies and Gentlemen,}
this is the main text
\closing{yours sincerely\\ \mbox{}}
\ps
Take care. this is postscript.
\encl{enclosures}
\cc{additional addressees}
\end{letter}
\end{document}