"Common wisdom on Linux has it that recompiling the Linux kernel
is the meanest test of a system administrator's skills. While it's
true that messing with kernel modules isn't trivial, there's a
higher peak: hacking sendmail's configuration file. This tip shows
you how to ruin your mail system with ease.
"Sendmail, that crusty and ancient (but also bulletproof) mail
program, relies on the /etc/sendmail.cf file. These days, that file
is usually generated using a heap of macros, much as a piece of
assembly code can be entirely coded in macros. You can, however,
hack sendmail.cf by hand, as long as you bear all the consequences.
In the end it's still a plain text file..."