Quelle: https://wiki.debian.org/UnattendedUpgrades

apt update
 
nano /etc/aliases
 
Add following line:
root:   hostmaster@domain.de
Save+Quit
 
newaliases
 
apt install postfix -y
>Satellite system
>FQDN
>SMTP relay host
 
apt install unattended-upgrades -y
dpkg-reconfigure -plow unattended-upgrades
<<Yes>>
nano /etc/apt/apt.conf.d/50unattended-upgrades
 
uncomment:
Unattended-Upgrade::Mail "root";
Save+Quit

Optional, postifx testen:

apt install mailutils -y
mail root
Cc:
Subject: Test
Hallo!
Ctrl+D
tail -f /var/log/mail.log

Oct 11 17:39:56 debian postfix/smtp[31370]: 8594940A46: to=<hostmaster@domain.de>, orig_to=<root@debian.domain.local>, relay=smtp.domain.de[1.2.3.4]:25, delay=0.14, delays=0/0/0.11/0.03, dsn=2.0.0, status=sent (250 2.0.0 Ok: queued as B19BF48C21)