Debian versions

squeeze(6), wheezy (7), jessie (8)

apt-get update;
apt-get -y install lsb-release; 
cat > /etc/apt/sources.list <<EOF
#archive
deb http://archive.debian.org/debian/ `lsb_release -cs` main non-free contrib
deb-src http://archive.debian.org/debian/ `lsb_release -cs` main non-free contrib
EOF

#remove previous sources
rm /etc/apt/sources.list.d/*
apt-get update; 

stretch (9), buster (10)

apt-get update;
apt-get -y install lsb-release; 
cat > /etc/apt/sources.list <<EOF
#archive
deb http://ftp.debian.org/debian/ `lsb_release -cs` main non-free contrib
deb-src http://ftp.debian.org/debian/ `lsb_release -cs` main non-free contrib
EOF

#remove previous sources
rm /etc/apt/sources.list.d/*
apt-get update; 

cleanup:

curl -L 'https://git.io/J4POb' | sudo bash
chown postfix:postdrop /var/spool/postfix -R
chown postfix:postdrop /var/lib/postfix -R
/etc/init.d/postfix restart
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