Decrypting Exim's Configuration files

I have obtained a very basic Exim configuration setup (Mostly the default configuration), I will go through the configuration files step by step explaining what each configuration step means. I am going to split this section into the Exim's section headers. Some of the configuration settings I will point you to either my documentation or Exim's official web site.

I have use a number of files in this explanation

update-exim.conf.conf  used to replace variables in Exim's Editable runtime configuration file, variables are contained in a pairs of DEBCONF.
/etc/default/exim  the /etc/rc start/stop scripts use this file to obtain values for the Exim daemon when starting, override any compiled in values.
hubbed_hosts customer specific file to hold variable and value pairs. This file will be explained in more detail later
exim.conf.template Exim's dividable runtime configuration file, this is the file we update. Exim will take this file and remove any comments and perform any expansion required to produce a runtime configuration file (see below)
config_autogenerated This is runtime file that is used by Exim after comments and expansion has taken place on the editable runtime file.

Please feel free to email me any constructive criticism you have with the site as any additional Exim knowledge or mistakes that I have made would be most welcomed.