I set up SSMTP with Gmail because I wanted to get mails for cron results.

I have these in /etc/ssmtp/ssmtp.conf:

root=<USER>@gmail.com
mailhub=smtp.gmail.com:587
#rewriteDomain=
hostname=gmail.com
UseTLS=YES
UseSTARTTLS=YES
AuthMethod=LOGIN
AuthUser=<USER>@gmail.com
AuthPass=<SECRET>
FromLineOverride=YES

Make sure this is only root-and-ssmtp-readable.

And /etc/ssmtp/revaliases:

root:<USER>@gmail.com:smtp.gmail.com:587
livibetter:<USER>@gmail.com:smtp.gmail.com:587

You can test with

echo mailbody | mail -v -s "mail subject" someone@example.com
echo mailbody | sendmail -v someone@example.com

I don’t have this mail command on my Gentoo, but it seems popular in every page I have read.

If you want to send a more complete test email via sendmail you can

echo -e "Subject: mail subject\nTo: someone@example.com\n\nmailbody" | sendmail -v someone@example.com