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Greylisting's email servers implement an anti-spam process known as Greylisting. The basic idea is make sure that the sender's email system complies with Internet standards for sending mail. If the sender's email server(s) are compliant, no problem. If sender's server(s) are not compliant (as is the case for most spam-bot networks), the mail will not be delivered.

Most legitimate email services are compliant, but if yours is not, your mail will not reach us. All the big free services such as gmail, yahoo, MSN, etc. are fully compliant.

Our auto-responder will send a reply if your mail makes it past the greylisting system. If you never receive the auto-reply, then something is wrong on your end.

Remember, you can always pick up the phone and give us a call if you have questions that can't be answered by the Huecotanks State Historic Site staff. Our number is
915-412-5769, we usually answer unless we are in an area with poor cell coverage.

The Geeky Details (this has been plagiarized from several web sources):

Greylisting is a cross between black- and white-listing, with mostly automatic maintenance. A key element of the Greylisting method is this automatic maintenance.

The Greylisting method is very simple. It only looks at three pieces of information (which we will refer to as a "triplet" from now on) about any particular mail delivery attempt:

  1. The IP address of the host attempting the delivery
  2. The envelope sender address
  3. The envelope recipient address

From this, we now have a unique triplet for identifying a mail "relationship". With this data, we simply follow a basic rule, which is:

If we have never seen this triplet before, then refuse this delivery and any others that may come within a certain period of time with a temporary failure.

Because the Simple Mail Transport Protocol (SMTP) was deliberately designed to compensate for unreliable transport, the possibility of temporary failures is built into the core spec (see RFC 821). As such, any well behaved message transfer agent (MTA) should attempt retries if given an appropriate temporary failure code for a delivery attempt (see below for discussion of issues concerning non-conforming MTA's).

During the initial testing of Greylisting in mid-2003, it was observed that the vast majority of spam appears to be sent from applications designed specifically for spamming. These applications appear to adopt the "fire-and-forget" methodology. That is, they attempt to send the spam to one or several MX hosts for a domain, but then never attempt a true retry as a real MTA would. From our testing, this means that in the test environment, based on a fairly conservative interpretation of testing data, we have attained an effectiveness of over 95%, and that is with no legitimate mail ever being permanently blocked.

In addition, with the recent rampant proliferation of email-based viruses, Greylisting has been shown to be extremely effective in blocking these viruses, as they also do not tend to retry deliveries. And since these viruses are fairly large, bandwidth and processing savings are significant versus the standard method of accepting delivery and local virus scanning.

This blocking comes with a minimal price from the terms of local resources. Assuming the use of a local datastore for the triplet and other metadata, there is no required network traffic caused by Greylisting other than that associated with the connection itself. Since we are not checking the contents of the message at all there is very little processing overhead, unlike many other spam blocking methods.

There is one effect that could be seen as either a positive or negative. Because the Greylisting method delays acceptance of unknown mail, a little more work is required for the sending MTA . But, it generates a lot more work for the spammer's systems, hopefully enough to make the costs of spamming higher, possibly even to the point of making spamming unprofitable for some of them.

The best part is that because greylisting will never permanently fail a message delivery, as long as the delivering MTA's are well behaved, greylisting will never cause legitimate mail to bounce.