SENDMAIL is a simple command line utility which can be used to send an email message. It's primary use is for batch/command files -- for example, you might include it in a batch file that performs a particular backup function and have SENDMAIL send an email to a particular person indicating that the backup had finished. Or even better, include some type of test as to whether the backup was successful or not, and send an email indicating that success (or failure).
You can specify all the email options (to address, from address, subject, message, etc.) on the command line, or you can pre-configure it to always use certain parameters (like the SMTP server to be used, for example). SMTP authentication and file attachments are supported. There is also an option to log all activity to a log file, so you can verify what emails were sent, as well as check for any errors that might have occurred.