spc476/x-grey

X-Grey: The Greylist Daemon.

This implements greylisting, as described in

	http://projects.puremagic.com/greylisting/

This is an anti-spam measure and has been shown to reduce the amount of spam
by 90% (at least, on the systems I've installed this program on).  This
program is different than most greylist implementations in that it keeps
everything in memory, and supports more than one MTA.

Features:

	* Fast.  It can handle around 6,000 sustained requests per second on
	  a 2.6GHz machine.  When tested on a 120MHz 486, it peaked out to
	  380 requests per second.  It's fast because everything about the
	  server is stored in memory.

	* Configurable.  You can have everything greylisted, or only mail
	  from one particular server, from a particular sender to a
	  particular recipient.  Or anything in between.  Email can be
	  accepted, rejected or greylisted based upon the IP address, sender
	  email or recipient email.

	* Dependable.  Configuration changes can be made while the server is
	  running, so you never have to shut it down.  If the program ever
	  crashes, it will restart itself, picking up where it left off.

	* Flexible.  It currently supports Sendmail and Postfix, and
	  extending it to support other MTAs is not hard.

	* Networkable.  Have more than one email server?  It can support
	  multiple MTAs (even different types) across a network (although it
	  can work over the Internet, it's best to keep an instance of the
	  Greylist Daemon on a local segment).

	* Boundable.  It consumes a predictable amount of memory and disk
	  space.  Expired tuples are automatically purged from the system.

Note:

	If you pulled the source code from Github, you'll need to run
	"scripts/post-commit" to generate the version.h file.  You can, of
	course, add this script to the ".git/hooks" directory to re-generate
	this file on each commit.

Acknowledgements:

	I would like to thank Pick Internet <http://www.pickint.net/> and
	Harmony Logic for funding the development of this program and for
	alpha testing this program.  I would also like to thank Gladesoft
	<http://gladesoft.com/> for feedback and keeping me honest in coding
	the program, and for running a beta test version of this program.