Almost all configuration information is read at runtime from a text configuration file.
Macro definitions (defining the value of macros used internally) are encoded.
Header declarations (the format of header lines that are specially processed and lines that are added or reformatted) are embedded.
Mailer definitions (with information such as the location and characteristics of each mailer) are included.
Name-rewriting rules (a limited pattern-matching system used to rewrite names) are defined.