By Jon Stewart
BBC Go Digital
If you have an e-mail account, you will almost certainly receive spam but are the measures to try and stop it actually blocking our legitimate messages too?
Spam is a problem for almost all email users
For some people receiving spam just means having to delete unwanted messages, for others it can make their account effectively unusable.
Figures from a global mail filtering company, MessageLabs, show that 73% of the 12.6 billion e-mails they checked during 2004 were identified as spam.
Often when you sign up with an internet service provider they provide an e-mail-filtering service, which may be automatically set to quarantine spam.
The "fine tuning" of that filter can mean the difference between getting too many messages offering to sell you things you never knew you wanted, and losing valid messages.
The problem is that identifying spam is not easy.
"We use a variety of methods," Paul Wood, chief information security analyst for MessageLabs, told the BBC World Service programme Go Digital.
"Very often we can signature spam in the way we can signature viruses.
"So once we have a signature of that particular spam, it can be applied to other e-mails and therefore maybe we can stop them because we have seen them before. It's a kind of 'fuzzy fingerprint'."
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These sorts of content filters can be very effective, but they can also filter newsletters, or other bulk e-mails to which you may have subscribed.
"We can also look at the header information. We can see the IP address that spam e-mail has come from - do we have any history about that address?"
The IP address is the unique number given to a computer when it goes online and security firms can look at this to determine if the originating computer has a track record of sending spam.
E-mail headers contain information on who sent an e-mail and when, and who it was addressed to.They also contain information on the chain of servers that were used in delivering it.
This information can be forged, which is a fairly sure indication the message is spam, and the sender does not want to be traced.
"If we look more closely at the actual content of the e-mail, if there's a link to a website, then do we know anything about that website?" said Mr Wood.
Other techniques spam filters use include a statistical analysis of keywords within an e-mail.
Bayesian filters calculate the probability of a message being spam.
It is a very robust way of filtering, and it is adaptable.
The filter is 'trained' by the user, who marks messages held in a quarantine area as 'spam' or 'not spam', and future false positives are reduced.
There are simple techniques too, such as black-listing and white-listing. All messages from a sender who is black-listed are blocked, or only messages from white-listed senders are allowed through.
Some very simple steps can be taken to help you avoid spam.
Many people set up two or more e-mail addresses. One, the 'real' one, is only given to people you want to hear from.
The other one is used on the internet, in chat rooms, on forums, anywhere spammers are likely to pick it up.
All messages sent to that account can then be ignored.
One of the golden rules regarding spam is never to reply to a spam message.
E-mail addresses they send to are often generated automatically, and replying will just confirm that your account is active.
The same often applies to any link within an e-mail offering to remove you from a mailing list. There is a good chance that it will actually do the opposite.
But it is a constant battle, as spammers are always finding ways around the filters, so it is a case of getting the balance right.
"That's the key problem anti-spam organisations are facing," according to Mr Wood.
"You can have very aggressive rules which mean you block 99.9% of spam, but what that means is you'll also block a higher proportion of the non-spam."
Do you have an ongoing battle with spam? Or have you found ways to combat the problem? Send your views to Go Digital.
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