If the program exits abnormally or produces any output, an error message is generated. As server administrator, you are less likely to run the program delivery module in trusted mode. This is because trusted mode bypasses some of the security systems that systems administrators use. For example, in trusted mode users can set up accounts with improper system permissions, because they can assign an arbitrary Unix login to any account. Such an account could then be used to run commands as the assigned user, provided the user has a valid shell.
Trusted mode can also open the doors to system invasion via the Internet.
When using trusted mode, you and the system administrator can take precautions that minimize the risks. Second, set up special accounts such as bin, sys, adm, and so forth with shells that aren't valid for delivering mail to programs. The messaging server feeds the message to the running program. If the program exits abnormally or produces any output, it generates an error message. The following instructions explain the steps that must be performed to enable program deliveries.