GreenNet uses an effective anti-spam method called greylisting. Occasionally it is alleged to cause delays and 450 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 (although complete spam detection rates are much higher). 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 causes bounced messages. 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, especially if you are expecting mail from a system that you have not received mail from before.
Greylisting technical details
If you get a response including
450 <domain.example.com [IP address]> : 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 ther 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 are 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.
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
If you get a 450 error when trying to send out through GreenNet, check that you have “SMTP authentication” set on with your username and password in your email program.
