The sendmail program offers a mode of operation (called rule-testing mode) that allows you to observe the flow of addresses through rule sets. The -bt command-line switch causes sendmail to run in rule-testing mode. This mode is interactive. You enter rule set numbers or names, addresses, and other commands, and sendmail processes them and prints the results. The -bt switch's chief use is in testing changes in the configuration file. It is also useful for learning how rules and rule sets work.