What is Greylisting?
Greylisting is a new method of blocking significant amounts of spam at the mailserver level,
but without resorting to heavyweight statistical analysis or other heuristical (and error-prone)
approaches. Consequently, implementations are fairly lightweight, and may even decrease network
traffic and processor load on your mailserver.
Greylisting relies on the fact that most spam sources do not behave in the same way as "normal"
mail systems. Although it is currently very effective by itself, it will perform best when it is used
in conjunction with other forms of spam prevention. For a detailed description of the method, see
The term Greylisting is meant to describe a general method of blocking spam based on the
behavior of the sending server, rather than the content of the messages.
Greylisting does not refer to any particular implementation of these methods.
Consequently, there is no single Greylisting product. Instead, there are many products that
incorporate some or all of the methods described here.
If you wish to contribute via a method other than Paypal, please contact me directly
Do you sell spam filtering software or services?
No, our fully functional example implementation is called relaydelay and is available for free
from the Downloads page. But we are happy to accept donations if you feel
that the research and/or software provided here are valuable, and you would like to support
our continued development. To donate, see the Credits page or the link above.
I received a bounce message about an email I sent, and it directed me to this URL. What's going on?
Please understand that this site is only giving information about the Greylisting method of blocking
spam. Many different pieces of software have implemented some form of the method described here, but this
site has no affiliation with the server that bounced your mail or with the people who run it. Some mail servers
may bounce some email with a message giving this website's address. This is only so that your mail system
administrator can find technical information about the method. We cannot do anything to stop your email from
getting bounced, it is up to you to contact your mail server administator and convince them to configure your
mail server to properly handle 45x error codes and delivery retries, which should solve your problem.
I run a mailserver, and some users say we are being blocked. How do I get off your list?
We do not run a blacklist. There is no centralized list to request checking of your server, or to request removal from.
The reason your mailserver is getting blocked is most likely that it does not properly handle 45x SMTP error codes, and does
not retry delivery within a reasonable amount of time after receiving such a code. Please read the whitepaper
for further technical information. Please, don't contact us asking for removal. Since there is nothing to remove you
from, any requests for removal will be ignored.
I run a mailserver, can I use Greylisting?
Yes, there are implementations of Greylisting available for many types of mail server software.
Even if our implementation (for Sendmail) is not suitable for your environment, there are many
others to choose from. See the Links page for links to available implementations.
Can I use Greylisting on my personal mail account?
Because Greylisting methods are designed to work at the mail server level, unless you have control of your own
mail server, or your ISP has installed a Greylisting implementation for you, you will not be able to
take advantage of Greylisting.
Where can I get help setting up filtering based on Greylisting?
Your best bet is to join the mailing list (see the Mailing Lists page)
and ask for help there. To avoid asking questions that have already been answered, please look
for answers in the mailing list archives first.
My Email provider uses Greylisting, and a few people are reporting mail to me is bouncing. What's happening?
There are a few mail systems that do not properly follow the email protocols as defined by the RFC standards specifications.
Due to this failure to follow the standards, bounces may be generated from their sending server when they shouldn't.
The most notable one is some older versions of Novell Groupwise. Chances are, the people who are getting back bounces
for the mail they send you are using a provider that is running one of these old broken versions of mail server software.
The best solution is to have the people getting the bounces get in contact with the system administrators of their email
system, and have them upgrade to a version that is standards compliant.
Non-compliant Mail Servers:
- Novell Groupwise 6.0 - Confirmation Link
- ISMail 1.7.1 and prior - Non-compliant. Reported as fixed in ISMail 1.7.4 and later.
- InterMail 4.0 - Reported
- Kerio MailServer 5.0.5 - Reported