\documentclass[DIN,
%               draft=on,
%               fontsize=12pt,
               a4paper,
               pagenumber=false,
               fromemail=true,
%               fromrule=true,
               ]{scrlttr2}

\KOMAoptions{
  paper=a4,
  fromalign=center,
  fromrule=aftername,
  backaddress=true,
  parskip=half,
  enlargefirstpage=true
} 

\SweaveOpts{strip.white=false}

\usepackage{ngerman}
\usepackage{times}
\usepackage{url}
\usepackage[utf8]{inputenc}
\usepackage[ngerman]{babel}
\usepackage{Sweave}

\setkomavar{fromname}{your name}
\setkomavar{fromaddress}{street and number \\ zip code and location}
\setkomavar{signature}{your name}
\setkomavar{fromemail}{your.mail@provider.com}
\setkomavar{fromphone}{XXX}
\setkomavar{place}{yourPlace}

%\setkomavar{subject}[subject: ]{Subject}
\addtokomafont{subject}{\sffamily}
\setkomavar{title}{title}

\let\raggedsignature=\raggedright                   

\begin{document}

<<echo=F,results=tex>>=
cat("\\begin{letter}{",fullName, "\\\\", adress1, "\\\\", adress2, "}")
cat("\\opening{", opening ,"}" )
@ 
here is a letter only for you. I swear, that this is no auto-generated
mass letter...

  \closing{Best regards,}
\end{letter}

\end{document}