GreenNet uses an effective anti-spam method called greylisting. Occasionally it may cause delays or even bounces emailing someone at GreenNet. However, in most cases this should be resolved at the sending server. For instructions on removing spam from incoming mail, see Filtering out spam.

Greylisting at GreenNet

Since April 2004, GreenNet has been using an innovative and effective method of improving the mail system’s ability to limit the amount of spam that is delivered to your mailbox. It works by identifying spam according to technical, rather than language-based, criteria. The effectiveness of this method is evident in our logs, which show that this measure alone prevents about 80% of spam messages being delivered. (Complete spam detection rates involving all all preventative measures are much higher; typical stats show the relative contribution of greylisting.) Theoretically, no legitimate mail should ever be permanently blocked.

The method, known as “greylisting”, is based on the simple observation that while correctly-configured, legitimate mail systems attempt to deliver messages repeatedly whenever there is a temporary delivery failure, most spam is delivered through special applications, which adopt a “fire-and-forget” method. So we have set up our servers to refuse the first delivery attempt by a new mail sender, and only accept the second and any consequent ones from the same sender to the same recipient. In this way, most spamming applications give up when sending mail to people with GreenNet email addresses.

Since implementing greylisting at GreenNet, we have discovered some cases of legitimate internet service providers who are not following the correct technical standards of email delivery, which say email servers should retry for “at least 4-5 days”. If a server wrongly gives up immediately, emails bounce. With your help we have developed the system to recognise these “faulty” senders. So if you have any questions or problems regarding missing messages do let us know, and as we can always use that information to improve the system.

Although the method has been designed to minimise its impact on end users, by its nature you may experience some delays in email delivery, typically of 10-20 minutes, if you are expecting mail from somewhere that you have not received mail from before.

Greylisting technical details

If you get a response including

450 : Client host rejected: Service is unavailable

then this is a result of a greylisting system and not normally a problem. 4xy codes are defined as temporary and mail servers should resend. However, if this comes back to you in a bounce message (Delivery Status Notification), then your mail server or the mail server of your service provider may need upgrading.

Many well-known email destinations, for example Yahoo, Greenpeace and the NHS, are known to use greylisting, so if you have problems with 450 errors when transmitting, these won’t be limited to GreenNet. According to a 2009 EU study, about 50% of European ISPs use greylisting for connections from at least part of the internet.

If you have problems contacting any addresses on GreenNet, please

1) For urgent messages or if you get a bounce, you may want to send the message again. By this time your message will be cleanlisted and the second message should go through immediately.

2) Ask your ISP to check their server(s) for retry times in response to a 450 message. There were known to be two buggy old mail server programs which do not comply with internet standards, Novell GroupWise 6.0 and InterMail 4.0. Please let your ISP know that if they still use one of these, emailing will be unreliable to an increasing number of other ISPs. Alternatively, it could be a misconfigured version of Exim. We have provided more technical information on making Exim configuration send to greylisting servers here. Another program, Mailgate, may have a “Fail after” setting as minutes rather than days.

3) Contact GreenNet by phone or email, giving the sender and recipient addresses and the date the email was sent or rejected through the other ISP. We can then investigate and if necessary, add the mail servers to our cleanlist.

Not being able to send out from GreenNet

If you are a GreenNet user and get a 450 error when trying to send out through GreenNet, check that you have “SMTP authentication” enabled, with your username and password, in your email program. We have information on how to enable SMTP authentication for several email programs.