Comment 27 for bug 1881196

Xenial verification:

$ lxc launch ubuntu-daily:xenial postfix-cert-xenial
$ lxc shell postfix-cert-xenial
# apt-get update && apt-get upgrade -y
# cat <<EOF >/etc/apt/sources.list.d/ubuntu-$(lsb_release -cs)-proposed.list
deb http://archive.ubuntu.com/ubuntu/ $(lsb_release -cs)-proposed restricted main multiverse universe
EOF
# apt-get update && apt-get install -y postfix/xenial-proposed
# postfix tls deploy-server-cert /etc/ssl/certs/ssl-cert-snakeoil.pem /etc/ssl/private/ssl-cert-snakeoil.key
postfix: Postfix is running with backwards-compatible default settings
postfix: See http://www.postfix.org/COMPATIBILITY_README.html for details
postfix: To disable backwards compatibility use "postconf compatibility_level=2" and "postfix reload"
postfix/postfix-tls-script: fatal: no private key found in file: