Implement greylisting feature

Greylisting (or graylisting) is a method of defending e-mail users against spam.

With this feature, qmail will "temporarily reject" any email from a sender it does not recognize. If the mail is legitimate the originating server will, after a delay, try again and, if sufficient time has elapsed, the email will be accepted. If the mail is from a spam sender, sending to many thousands of email addresses, it will probably not be retried.

We 'll make use of Perl scripts jgreylist and jgreylist-clean written by John Simpson

Edit /var/qmail/supervise/qmail-smtpd/run and change the following

OPTIONAL : If you need more performance you can use a compiled version of jgreylist

List of optional features