sSMTP is a simple SMTP client for sending mail from your local client to a mail hub with a more serious MTA. For example, my main server is running Postfix, but I want to send mail from my laptop. I use sSMTP to do this.
Before you use sSMTP, you need to configure it so it knows how to
connect to the mail hub. My
root=jdoe mailhub=mail.example.net rewriteDomain=example.com
This sends mail via
mail.example.net, and replaces my current domain
firstname.lastname@example.org will recieve all
system-generated email (i.e. cron job output).
There are other settings available in the config file. The stock
config contains lots of comments explaining how to use them, and the
ssmtp.conf man page contains even more details.
If sSMTP doesn't scratch your itch, the next level up in terms of features is msmtp, which I used to use before I setup my own mail hub.