We are using one of the most advanced SPAM filters on the internet. Anti-Spam SMTP Proxy Server (ASSP) is an Open Source project that utilises dozens of advanced spam mitigation techniques. It continually adapts itself to detect spam, and also allows email users to adjust its behaviour.

How it works

Each time a remote computer connects to the Air-Stream mail server, ASSP starts to build a profile of that connection. This involves analysing many technical details about the SMTP session and keeping a ‘score’ based on how many suspicious things are observed.

Sometimes a connection is rejected immediately, for example if the remote computer is found to be listed in a ‘known blacklist’ of Internet addresses. Otherwise ASSP uses the score for the incoming message to determine whether or not the message should be flagged as spam, based on predefined thresholds. ASSP keeps track of these scores and uses them to refine it’s own internal databases for more accurate detection in the future.

When the incoming message is eventually delivered into your INBOX, the message ‘headers’ contain a bit of extra information about what ASSP discovered and what action was taken. Email users can then leverage this information, for example to configure their client software, or to better fine-tune the spam filtering process.

Things You Should Know About

The whitelist

The most important ASSP feature to understand is the ‘whitelist’. Emails from addresses that are on the whitelist will bypass most of the rigorous spam tests, so that prompt delivery is guaranteed.

Each time you send an email TO someone using the Air-Stream mail server, their email will be whitelisted automatically. Obviously for this reason you should NEVER reply to a message that is flagged as spam, or that you suspect might be spam.

To add new addresses to the whitelist, send an email to assp-white AT air-stream.org with the email addresses listed, one per line, in the body of the message. You will receive a confirmation message from the system to let you know that

Greylisting

Greylisting is a technique which delays an incoming message from an unknown source for a short, random period (usually a couple of minutes) and gives the remote computer a ‘temporary failure, please try again later’ message. This technique is highly effective in preventing spam from automated software such as a virus on an infected computer, since usually these bots will give up rather than retry the connection later.

This means you may occasionally experience a delay when you are expecting a message from a sender that is not yet whitelisted. If the sender is known, you might want to add them to the whitelist ahead of time to avoid the delay, otherwise you can do this once their message arrives.

Read more about greylisting: http://en.wikipedia.org/wiki/Greylisting

Training

ASSP does a great job, but occasionally a spam email slips through the system. If you notice this happening and would like to correct the behaviour, you can report it to ASSP for re-training using the following procedure:

To report a spam, simply forward the message ‘as an attachment’ to assp-spam AT air-stream.org. If your email software doesn’t support ‘forward as attachment’, simply forward the mail.

Next time the spam database is analysed, adjustments will be made based on your reports.

False Positives

A false positive is a legitimate email that has been flagged as spam. If you notice this happening and would like to correct the behaviour, you have a couple of options.

Firstly, you can add the sender to the whitelist using the procedure outlined above. This will ensure they are no longer analysed.

Alternatively, you can offer to retrain the ASSP spam filter by forwarding the message ‘as an attachment’ to assp-notspam AT air-stream.org. If your email software doesn’t support ‘forward as attachment’, simply forward the mail.

Next time the spam database is analysed, adjustments will be made based on your reports.

The SPAM Folder

You will notice your Air-Stream INBOX has an automatically generated subfolder called SPAM. Any messages that have been flagged as spam by ASSP will be automatically moved into this folder. You should check the folder periodically for ‘false positives’.

Note: This is not applicable if you are using the forwarding feature to forward all Air-Stream email to another external account.

Summing Up

Just to remind you of the basic operation of our spam filter:

Anyone you email will never have a message blocked.
Never email a spammer, you will falsely validate their address, authorising them to spam us further.
Don’t forward spam, it makes the spam filter think we like it.
Report spam by forwarding it to the assp-spam AT air-stream.org address.
Incoming messages will never be blocked, they will either be delivered to your INBOX or the SPAM folder within.