On the presupposition that most people have no interest in running an e-mail server for sending/receiving e-mails but probably wouldn't mind being able to send e-mails easily from the command-line, let's set up a way to use a public mail service like Gmail.
First of all, you'll need mailutils, which is probably installed, but if not:
# apt-get install mailutils
From this point forward, there are a lot of choices, and they all seem to do more or less the same thing, so I've chosen ssmtp.
# apt-get install ssmtp
As root, add the following to /etc/ssmtp/ssmtp.conf:
and uncomment/add the line (which may or may not be useful, but I noticed that I've done that):
If not using Gmail, you're going to have to figure out the SMTP server address yourself. Depending on what you choose to do this for, you may also want to create a separate account for such purposes. For example, I've given my Raspberry Pi its own Gmail account, which occasionally sends my account e-mails about stuff.
The way sending mail works is that the user uses the mail command, which in turn calls the sendmail command, which should be a symlink to ssmtp. Make sure this is the case:
$ whereis sendmail
sendmail: /usr/sbin/sendmail /usr/lib/sendmail /usr/share/man/man8/sendmail.8.gz
$ ls -l /usr/sbin/sendmail
lrwxrwxrwx 1 root root 5 Jul 10 2012 /usr/sbin/sendmail -> ssmtp
If not, change it so that it is (by hand or with update-alternatives or similar).
At this point, everything should just work:
$ echo "Hello, World!" | mail -s "Test Subject Line" email@example.com
I use this within scripts/cron jobs for when things go wrong, and I should be informed.