When your machine boots, it executes the commands in one or more Bourne shell scripts. Examine the script that runs the sendmail program in daemon mode. Write a small Bourne shell script that checks for the existence of the echo(1) program and then executes that program. Model it around the way sendmail is run.
For each program that is mentioned in your configuration file's
M lines (like /bin/mail and /bin/uux),
read the appropriate online manual page.
Examine the possible command-line arguments to determine how
envelope information is passed. Compare what you discover to
A= part of each corresponding
Locate all the files mentioned in
O lines in your configuration
file. For any that exist, what kind of file is it: data or text?
If it is text, read it. What purpose do you suppose it has?