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How Our Anti-Spam System Works


The good news is that the war against spam is being won, but there are plenty more battles left before the enemy finally gives up.


Our emphasis in fighting spam is not to lose any genuine emails to "friendly fire".

We use a variety of different techniques to rate the likelihood of an email being spam.
Each technique produces a score and the scores are added up to give a Spam Index.
If this is 20 or greater the email is considered "definite spam" and deleted.
(I've never seen a genuine email be miss-classified as definite spam.)
If the Spam Index is 5 to 20 it is considered "probable spam" and sent to a folder where you can choose to examine it.
Probable spam over 7 days old is automatically deleted.
Email with a Spam Index under 5 is considered to be genuine email and sent on its way, unchanged, to your Inbox.

For an individual account we normally send probable spam to the Junk E-mail folder.
Outlook doesn't allow this folder to be deleted so it might as well be used for this purpose.
This goes against the principal of deleting Spam at the server but it's very convenient.
We recommend that you turn off Outlook's own Junk E-mail Filtering system otherwise, when a mistake is made, you don't know which system made it.
To disable spam filtering in Outlook go to:-
Tools - Options… - Preferences - Junk E-mail… and set the options as shown below:-


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Disabling Outlook's own spam filtering system  


For group email accounts, we can send everybody's probable spam to a single Public Folder where one volunteer can screen it for false positives on behalf of the whole group.

Spam is sent by very clever people who are constantly innovating and changing tactics to circumvent all the anti-spam measures arraigned against them, in a similar way to how the 'flu virus constantly mutates to defeat the body's defences.
Our anti-spam system is pretty good but a few spam emails will get through to your Inbox and occasionally some new spammer's trick will catch us out for a short while until our system adapts.

We're constantly monitoring and reviewing our anti-spam system to maintain its effectiveness and have several "honey trap" accounts that collect nothing but spam and so are sure to quickly attract any new strain of spam that emerges.

We refuse to adopt the heavy-handed anti-spam approach of some companies which leads to a larger proportion of genuine emails being lost, but our system will get it wrong sometimes and miss-classify genuine emails as spam.
To allow for this situation, our system forwards-on any email it thinks is spam but isn't absolutely sure, to a special folder for you to review.
For a human, a genuine email is easy to spot.
If you don't want to be bothered reviewing this folder it won't grow out of control, as an automatic process deletes emails here that are over 7 days old.



Forwarding Miss-Classified Emails Back To Us

The part of our spam filtering system that uses Bayesian Filtering benefits greatly from being told when it's made a mistake.
If you find a genuine email among your Probable Spam you should definitely forward it to us for re-feeding into our spam filter.
If spam emails make it through to your Inbox, however, the correct action to take is not so clear-cut. Read our advice on what should you do here.
For them to be of value, miss-classified emails need forwarding to us in a special way - instruction are here.



Initiatives to Reduce Spam

There are currently several different modifications to how the Internet's email system works that have been proposed in order to, more easily, identify which emails are spam.
Each proposed system has it's own group of "big players" that are advocating it and also trialling it.
The problem is that none of these proposed systems are perfect and so have only been implemented by a small proportion of the Internet's email servers.
Sender Policy Framework (SPF) is, so far, the most widely adopted standard; next is Domain Keys Identified Mail (DKIM).
At Arrowmail we make sure that all our outgoing email conforms to these standards in order to give them the best chance of being delivered but we don't use an incoming email's non-compliance with these standards as a factor to increase it's Spam Index.

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