15.7 Things to Try

• A file of rules and addresses is valuable as a tool for testing new and revised versions of a configuration file. Develop a small shell script that feeds such a file to sendmail in rule-testing mode, then filters the output to display only the returns of interest. Is it possible to test rules other than those that select a delivery agent?

• Is it possible to protect the queue directory with narrow permissions (such as 0700), yet still allow ordinary users to run sendmail in rule-testing mode?

• Is it legal to have an MX record point to itself? What about having an MX record pointing to a CNAME? What are wildcard MX records, and what pitfalls might you expect to encounter in using them?

• Consider a hub machine that is connected to two different domains. What rules might it have to include to recognize itself (or its clients as itself) in either domain? Remember that some versions of sendmail allow multitoken class matches, whereas other don't. What is the most portable way to recognize oneself in two domains?

• If a client is hybrid (a client in the hub/client scheme, that still needs to be able to receive mail), what changes will be required? Does anything need to be added to the client.cf file? Will rule set 0 need to be modified to select the local or prog delivery agents? Will a daemon still have to run?