I do a lot of work involving PHP-based web development. I often find myself needing to test e-mail functionality of a complex PHP application (such as Drupal), but I want to be sure that emails won't be sent to any of the unsuspecting users in the database of the application I'm testing.
Do accomplish this, I make a small but important change to my php.ini (actually in my conf.d/dev_mail.ini -- but it's the same thing):
sendmail_path="/usr/sbin/sendmail -i firstname.lastname@example.org"
This tells PHP to use this command line whenever the mail() function is used to send an email. PHP's default here is "sendmail -i -t". The -t tells sendmail to scan the message text for To:, Cc:, and Bcc: headers to determine where to deliver the mail. By eliminating the -t we tell sendmail to use the email address(es) on the command line to determine delivery, thus making sure that no matter what PHP's mail() tells sendmail, it will deliver to the address you want.