Once you hit the build it button, the forward will be created and you will be returned to the main configuration screen which will show your new forward, and now the rest of the columns and buttons will make some more sense! We have used "first.last" as the name of the forward, and some.mailbox@example.com as the destination address in this example.

Now that there is at least one forward, the update and delete links show up. They are used the change and delete forwards.

The rest of the options on the screen (e.g. most of them) are for controlling the two spam-filtering systems.

Spam Filtering

Enabling spam filtering is quite simple. Once forwards have been created, they will be shown with check boxes to control how the content filtering (spam preference column) and challenge-response (challenge preference column) systems are configured.

The default settings for both the content filtering and the challenge response systems is "off".

Content based spam filtering

We use spamassassin for content filtering. If it is enabled, the system will look at each message and assign it a spam score based on aspects of the message it thinks look like spam. If the total score is greater than the threshold (which defaults to 5), then the message is classified as spam.

If the content filter is set to "bounce", and spam is detected, then the message will be bounced with a "no such user" message (if the senders mail system actually displays the full bounce text, there will be a "spam detected" message inside the bounce).

If the content filter is set to "mark" and spam is detected, then the message will be wrapped inside a covering letter. The covering letter will have a subject starting with "**SPAM**" followed by the orginal subject line. The contents of the message will be a terse explanation of why the system thought the included message was spam. The combined message will then be forwarded as normal. (If challenge response is active, then the message will be forward to the challenge response system.)

Tuning the content-filtering system

We have enabled a number of tuning options for the spamassassin content filter. If you click on either the 'configure' link for a user, or the 'Global Spam Options" button at the bottom you can change some preferences for either that user, or for all users at the domain. Configurable items include the spam threshold as well as the blacklist and whitelist of addresses. (Blacklisted addresses are always considered spammers and white-listed addresses are never considered spammers.) Beyond that screen is an 'Advanced SpamAssassin Preferences' screen which can be used to do things like change the Spamassassin internals, like weighting's of rules, but that is far beyond the scope of this document.

Challenge response filtering

This section will talk about configuring the challenge response system, see this page for a higher level description of what the challenge response system does and how it works.

There are three main settings for the challenge response system. Setting it to "off" disables it. Setting it to "on" will cause it to run as a standard challenge response system by queuing inbound messages from unrecognized senders and sending a challenge back to the sender. Setting it to "warn" will behave similarly to "on" except that instead of challenging the sender, a "warning" message will be sent to the recipient stating that the system has queued a message. With either "on" or "warn", messages from white-listed senders will be delivered immediately, and messages from blacklisted senders will be bounced (e.g. thrown away and the sender notified).

C/R queue handling

Messages from unrecognized senders are queued, and that queue is visible if you click on 'configure' under the 'Challenge Preference' header. The queue display screen shows all senders with queued messages, the subject from the first queued message for each sender and more info. The screen has Accept and Reject buttons, which allow you to reject (blacklist and delete) or accept (whitelist and deliver) messages from the checked sender(s). There is also a Detail button for each sender which allows you to see all the subjects (if there is more than one) and provides the same type of Accept/Reject logic. It also has Delete and Deliver buttons which either delete or deliver the messages, but do not affect the blacklist or whitelist.

Tuning the challenge response system

There are some "issues" with challenge response systems, like handling bounces, and offensive subject lines. If you go into the queue display screen, and then click on 'Edit Challenge Response Options' you will be shown a preferences screen which allows you to control the white and black lists, as well as what the system will do with bounces (it can drop/ignore them, queue them, or deliver them).

That preferences screen will also allow you to control whether warning messages have a copy of the subject line from the queued message, we have provided this option because there are message out there with very offensive subjects, and if this system is being used to screen out such content... then we shouldn't forward it! We think this is most applicable to parents handling the queue for children accounts but there are probably other uses too!

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