sending mail after successful backup

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

Make sure your config in /etc/msmtprc looks something like this:

# Set default values for all following accounts.
port 587
tls on
tls_trust_file /etc/ssl/certs/ca-certificates.crt
account gmail
from <user>
auth on
user <user>
password <your password>
# Set a default account
account default : gmail