The include statement is a special statement that causes inclusion of a named file. It has the following syntax:
include file;
If file names a regular file, the contents of this file is included in this point. Otherwise, if file names a directory, Mailutils searches in that directory for a file whose name coincides with the name of utility being executed, and includes this file, if it exists.
It is a common approach to end the site-wide configuration file with an include statement, e.g.:
include /etc/mailutils.d;
This allows each particular utility to have its own configuration file. Thus. imap4d will read /etc/mailutils.d/imap4d, etc.