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Junk Email Current Blocking Action How the blocks work

All email to and from campus goes via a central mail hub (also known as a central mail server or central mail relay). As all mail is received centrally, it is possible to apply blocks at a single point which protect the entire campus from junk email. The main three lists used for blocking use different criteria and an explanation of each is given below. The three lists currently used are maintained as one RBL+ list at www.mail-abuse.com where much more information is available on the subject. These lists are accessed using the JANET RBL+ list which is a mirror of the central list. Other lists are in use operated by different organisations using slightly different criteria. We do operate a lookup service which displays information about which blocking lists a particular IP address is in, if any, at http://mailhub.bham.ac.uk/ebl The names of the companies used below are fictitious.

RBL

This is a list of addresses which have been a persistent source of junk email. They define their list as follows.

The MAPS (Mail Abuse Prevention System LLC) RBL (Realtime Blackhole List) is a list of hosts and networks which are known to be friendly, or at least neutral, to abusers of those networks either to originate or relay spam or to provide spam support services.

We are not aware of any legitimate email that has been blocked by using this type of list. .

For example, if the fictitious company "Hollywood Molls inc" send their email via the servers at "Cyberjunk Email Services", and this was listed as a known source of junk email, we would reject the email.

It is most unlikely that this list would cause any legitimate email to be rejected. Some companies use their own servers directly to send junk email, and these servers can end up in the RBL and are subject to blocks using the same mechanism.

DUL

The DUL is a list of known dial up addresses. Legitimate email is sent using an ISPs mail servers and not directly from the addresses assigned to systems which dial up, so should not be affected.

For example, if the company were to use a dial-up account at the fictitious company "Artists On Lines" and send the email directly which is how a typical bulk email package works, the email would not be accepted by us if the originating address was in the DUL.

However, legitimate dial-up users would use one of their ISP's mail servers to send their email so these users would be unaffected (the mail relay is specified as the smtp server in the mail client in use - for example Microsoft Outlook Express). It is believed that junk coming in through routes like this is commonly sent by using temporary free trial accounts.

Legitimate users who send email through the correct channels are much less likely to abuse the system as accounts are normally terminated once they have been used to send junk email. Furthermore it is possible to implement further anti junk email controls on the ISP's mail server, perhaps limiting the number of recipients to a reasonable number.

It is unusual for this list to cause any problems with legitimate communication, although people have occasionally misconfigured their system to send mail directly to remote sites over a dial up line. This is easy to do if they are running something like Windows NT with exchange mail server software instead of a normal mail client that most dial up users would run. If legitimate mail is rejected in this way, the sender needs to to reconfigure their mail software correctly to use their ISPs mail server as a relay. If the address is not a dial-up address, the sender should get their ISP to contact the maintainers of the list to get the address taken out of the DUL list. Contact details are on the Web page ( www.mail-abuse.com ).

RSS

This list is a list of open relays which have been used to send junk email These mail relays are either belong to ISPs and companies and allow anyone in the world to send email to anyone else. They have normally been left in this state accidentally, and most responsible companies and ISPs will rapidly close these once the problem has been drawn to their attention. If the open relay is closed, they can go to the RSS web pages and request that it is re-tested and removed from the RSS list.

For example, if the fictitious company Yo-Net are running an open relay and the mail was sent through that relay, which was in the RSS list we would refuse the email.

This is now one of the most common ways of sending junk email (spam) and using the RSS list is an effective measure in combating it. However, all email from Yo-Net's open relay would be blocked including legitimate email messages from their customers. The first action that the legitimate sender blocked by this action can perform is to contact their ISP's support desk or, if it is a company mail server, contact whoever is responsible for running it. In most cases the problem can be fixed quite rapidly and they can request that the server is removed from the RSS. In other cases an alternative mail relay may be suggested.

Last Updated 11 October 2005. Please mail any comments to C.B.Bayliss@bham.ac.uk