It will send an email if you uncomment this line and set it to "always". You dont have to use these sed commands, you can just edit the file. I'm just sharing them because I use several VMs:
sed -i 's,//Unattended-Upgrade::MailReport "on-change";,Unattended-Upgrade::MailReport "always";,' /etc/apt/apt.conf.d/50unattended-upgrades
Then set it only on error:
sed -i 's,Unattended-Upgrade::MailReport "always";,Unattended-Upgrade::MailReport "only-on-error";,' /etc/apt/apt.conf.d/50unattended-upgrades
Or to set it first to only on error:
sed -i 's,//Unattended-Upgrade::MailReport "on-change";,Unattended-Upgrade::MailReport "only-on-error";,' /etc/apt/apt.conf.d/50unattended-upgrades
Then run it manually with unattended-upgrades and it will send a "SUCCESS" email even if there are no updates to upgrade.