F command, rather than specifying the name of a file, specify
the name of a directory or a device such as /dev/null. Does
sendmail care? How does it handle (or not handle) nonfiles?
Use /usr/dict/words as a class file. Does reading a huge file slow sendmail down noticeably? Does this suggest that a database might be more efficient for some needs?
Are there security concerns surrounding use of the
F command? Imagine ways in which a cracker might exploit a careless use of the
F command to compromise your system. Skip ahead to
Section 22.4.2, "The F Command - File Form" to explore this question further.