Debian repositories are signed with PGP/GPG. If package installation or updates result in a GPG errormessage, you should first install the latest GPG public key.

To do this, enter the following command on the console.

cd /etc/apt/trusted.gpg.d
wget https://www.benno-mailarchiv.de/download/benno-mailarchiv-archive-key-40.asc

After that you can

apt-get update
apt-get dist-upgrade

Run to install the latest version.