install ssmtp in Debian buster

apt install msmtp

ssmtp

Package is currently unmaintained

This package has been orphaned since 2019-03-19. msmtp can be used as an alternative.

debian wiki: msmtp

msmtp: documentation

manpage msmtp

ssmtp is currently unmaintained, but you can install and configure msmtp as well:

For gmail (with deactivated 2-factor login) use this configuration in /etc/msmtprc:

# 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

To test it run

echo -e "Subject: msmtp test\nhello test." | msmtp _recipient_address_

You should find your sent e-mail in the recipient's inbox shortly afterwards.