This topic helps you to install postfix 2.5.1 on debian etch

Edit the /etc/apt/sources.list file

add these new repositories :

deb http://www.backports.org/debian etch-backports main contrib non-free
deb-src http://www.backports.org/debian etch-backports main contrib non-free

Optional: 

deb http://volatile.debian.org/debian-volatile etch/volatile main contrib non-free

Do apt-get update

Install postfix 2.5.x by the command line:

apt-get install -t etch-backports postfix postfix-ldap