In case you want cron to tell you that your backup job has been finished you can use MSMTP on Debian Buster. SSMTP is not maintained anymore.
echo -e "Subject: msmtp test\nhello test." | msmtp firstname.lastname@example.org
Make sure your config in /etc/msmtprc looks something like this:
# Set default values for all following accounts. defaults port 587 tls on tls_trust_file /etc/ssl/certs/ca-certificates.crt account gmail host smtp.gmail.com from <user>@gmail.com auth on user <user> password <your password> # Set a default account account default : gmail