Spam filtering at NTNU

What is spam?

Spam is unsolicited email, often advertisements for viagra or for financing possibilities in Nigeria. Spam is a problem because it is sent out in increasingly large amounts, and it is often difficult for the receiver to block out.

ITEA offers free spam filtering to faculties, institutes, student organizations and other users of NTNU's network.

Our solution is based on Spamassassin. Spamassassin assesses an email using various tests. The tests exploit the fact that spam usually differs from normal email in a variety of ways. Each test results in a certain value, and the total of these values is assigned to the email message as the spam level of that message.

How to filter spam?

If you are receiving email through NTNU's central email system, your messages are already filtered. Email most likely to be spam, will not be delivered to you, whereas email that has a certain probability of being spam, is tagged (by changing the subject field). You may change the behaviour of email delivery to you, by changing your spam level (see below).

If you are not receiving email through NTNU's central email system, system administrators of your subdomain (typically institute.ntnu.no) may use ITEA's solution (the link only exists in Norwegian) for tagging spam (without having to change their own systems to a degree worth mentioning). Contact orakel@ntnu.no if you do not know whether your email is delivered through NTNU's central email system or not.

What does NTNU's spam filtering imply?

Short explanation: you receive less spam. More in depth, your email will be automatically filtered by our email system.

There is a small chance of email being tagged as spam, even though it is not spam. Typically, this could be messages from medical students or employees talking about "viagra" or other subjects that often occur in spam. If you are afraid of losing email, you may change your personal spam level (see below).

ITEA constantly watches the spam level used, and changes it if it is needed. We recommend all 'normal' users to use the standard level.

Spam filtering levels

To make spam filtering easy for normal users, user settings are splitted into four different sensitivity levels. If you are not satisfied with the default setting High, you may change the spam level. Note that when talking of spam value, a higher score means a higher probability of a message being spam.

Extra high

This level refuses delivery of all email with a spam value above 4.0. If you choose this level, all messages previosly tagged as ***SPAM*** will never be delivered. It is up to you to decide if this is a setting you want to use.

High

This is the basic setting for all users, unless they have chosen something else.

This level refuses all email with a spam value higher than 8.0. Email with values above 4.0 are tagged with ***SPAM*** in the subject field.

Medium

This setting is for more experienced users and users wanting to receive emails that are very likely to be spam.

Email with a spam value above 12.0 will be refused delivery, messages with values over 8.0 will be tagged in the subject field.

None

No filtering. This setting is for users with an email client that does its own filtering, and users who want to receive all email.

Messages with a spam value above 12.0 will be delivered, and no messages will be tagged as spam in the subject field. The X-Spam headers are included, though.

For all levels, seperate X-Spam headers will be inserted into messages. These headers reveal which test that got a "hit" for the message, together with the calculated spam value. These headers can be used to do a more fine grained filtering, for instance to sort spam into seperate folders.