Postfix StartSSL

Generate certificate request

mkdir -p /root/ssl
cd /root/ssl
openssl req -new -newkey rsa:4096 -nodes -keyout mail.nindl.net.key -out mail.nindl.net.csr

Generate mail.nindl.net.crt with the respective .csr on startssl.com and copy to /root/ssl/mail.nindl.net.crt

Download the StartSSL Bundle

wget https://www.startssl.com/certs/ca-bundle.pem -O startssl-ca-bundle.pem
wget https://www.startssl.com/certs/sub.class1.server.ca.pem -O startssl-sub.class1.server.ca.pem
cp /root/ssl/startssl-ca-bundle.pem /etc/ssl/certs/startssl-ca-bundle.pem

Copy keys

cp /root/ssl/mail.nindl.net.key /etc/ssl/private/postfix.pem
cp /root/ssl/mail.nindl.net.crt /etc/ssl/certs/postfix.pem
cat startssl-sub.class1.server.ca.pem >> /etc/ssl/certs/postfix.pem

in /etc/postfix/main.cf

smtpd_tls_cert_file=/etc/ssl/certs/postfix.pem
smtpd_tls_key_file=/etc/ssl/private/postfix.pem
smtpd_tls_CAfile=/etc/ssl/certs/startssl-ca-bundle.pem