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.
Other email providers, for example Yahoo, are known to use greylisting, so if you have problems with 450 errors when transmitting, these won’t be limited to GreenNet.
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.
Spam labelling
GreenNet email accounts also come with a tool that tries to recognise unwanted email messages (so called ‘spam’) and labels them accordingly. In general we cannot prevent spam by deleting messages from known spam sender addresses as they keep changing their email addresses and the mail servers they (ab)use. We also have to be cautious as the tool can sometimes misidentify email as ‘spam’ (also known as giving ‘false positives’). False positives are very rare, and a high priority for us, so please report them. And so with the exception of obvious spam which we feel confident about rejecting, we quarantine most spam messages (effectively discarded after seven days), and in cases of doubt we label the messages for you to then filter out as they come into your mailbox.
Filtering
That is why GreenNet recommends setting up ‘Spam’ filters in your E-mail program. Filters (sometimes also called ‘Filter Rules’ or ‘Mail Filters’) allow your E-mail program to automatically detect messages which GreenNet servers marked as ‘Spam’ and move them to your trash box or junk mail folder. This also gives you a chance to look at them and check whether any of them were misidentified.
Setting up a Filter
Follow the links below for instructions on how to set up a’Spam’ filter in your E-mail program (if your E-mail program is not listed please send an E-Mail to support@gn.apc.org and we will add instructions for it as soon as possible).
Sadly Outlook Express 6 does not support filtering based on special header fields. We can, if neccesary, alter the subject line of suspected spam to assist filtering. Contact support@gn.apc.org if you require this.
Check the GreenNet Forum to find more and to add your own instructions.
Further Reading
There are historical discussions here – I get so much SPAM – about some of the things GreenNet is doing against spam.
GreenNet’s anti-spam tool is powered by a forked version of the open-source program SpamAssassin. Technical information is available there.
Comments
Post new comment