\documentclass[margin=1cm, varwidth, tikz]{standalone}
\usepackage{etoolbox}
\input{common_options.tex}
\input{common_packages.tex}
\input{common.tex}
\usetikzlibrary{calc}

\newcommand{\clientToServer}[2]{%
  \path ($(client) - (0, #1)$) edge[above] node[fill=red!20, minimum width=8cm] {\psolution{#2}} ($(server) - (0, #1)$);%
  \draw[->] ($(client) - (0, #1)$) -- ($(server) - (0, #1)$);%
}
\newcommand{\serverToClient}[3]{%
  \path ($(client) - (0, #1)$) edge[above] node[fill=blue!20, minimum width=8cm] {\psolution{\ifnumless{#2}{400}{\textcolor{green!70!black}{#2}}{#2} #3}} ($(server) - (0, #1)$);%
  \draw[<-] ($(client) - (0, #1)$) -- ($(server) - (0, #1)$);%
}

\begin{document}
\begin{tikzpicture}
\node[fill=red!20] (client) {Client};
\node[fill=blue!20] (server) at (8, 0) {Server};

\draw[] ($(client) - (0, 1)$) edge[above, ->] node {\textcolor{gray!80}{Verbindungsaufbau}} ($(server) - (0, 1)$);
\serverToClient{2}{220}{[Server Infos]}
\clientToServer{3}{HELO \textless{}domain\textgreater}
\serverToClient{4}{250}{[greetings]}
\clientToServer{5}{MAIL FROM: \textless{}sender\textgreater}
\serverToClient{6}{250}{OK}
\clientToServer{7}{RCPT TO: \textless{}recipient\textgreater}
\serverToClient{8}{250}{OK}
\clientToServer{9}{DATA}
\serverToClient{10}{354}{End data with \textless{}CR\textgreater\textless{}LF\textgreater.\textless{}CR\textgreater\textless{}LF\textgreater}
\clientToServer{11}{E-Mail}
\clientToServer{12}{\texttt{\textbackslash{}r\textbackslash{}n.\textbackslash{}r\textbackslash{}n}}
\serverToClient{13}{250}{OK}
\clientToServer{14}{QUIT}
\serverToClient{15}{221}{OK}
\draw[] ($(client) - (0, 16)$) edge[above, <-] node {\textcolor{gray!80}{Verbindungsabbau}} ($(server) - (0, 16)$);

\draw (client) -- ($(client) - (0, 16)$);
\draw (server) -- ($(server) - (0, 16)$);
\end{tikzpicture}
\end{document}