Sendmail: How does one blacklist annoying spammers?

[Greg Woods]

On Sun, 2010-06-27 at 08:24 +0800, Ed Greshko wrote:

> A well written greylisting milter will utilize a database to maintain a
> list of sending MTAs that have retried. 

Of course. However, many large sites (including ours, which is only
medium sized) have multiple IP addresses that send out mail, which
results in the same sender getting greylisted multiple times.

>  Additionally, the good milters
> will have the ability to specify whitelists and blacklists. 

Specifying them is one thing, maintaining them is another. Static
blacklists are useless for the reasons already stated (the sending IP
addresses of the spammers change too rapidly). White lists could be (and
are) used, but until someone actually has a problem, you can't know what
has to be whitelisted. In the several years we have been using
greylisting, only once have I actually had to whitelist a sender
(because it was some graduate student in Italy using a homegrown mail
sender that didn't have retry capability; the scientist here is not
interested in hearing about how the sender is violating several RFCs )-:

At any rate, the point is that greylisting *does* cause *some* delays. I
am NOT saying it shouldn't be used, in fact quite the opposite. I *am*
saying that someone looking to implement greylisting should be aware
that it will cause some legitimate mail to be delayed.

--Greg