Wednesday, July 23, 2008

Knowledge is Power Part I - AOL Spam Filters

Let me try and shed some light on AOL’s basic anti-spam architecture as a process flow and how it affects inbound email. There are three main types of spam filters that AOL uses i.e. blacklist filter, reputation filter, and a whitelist filter.

A blacklist filter is when an email is checked against AOL’s internal blacklist. If the sender domain is blacklisted, the mail will be blocked. The reputation filter then reviews the reputation metrics in the form of “abuse data” (spam complaints and spam traps). Once it passes the two filters, the domain is checked by a whitelist filter where it runs a check if the from-address (consisting of the domain) is in the recipient’s address book or if the IP and Domain are on any of their whitelists. Being on AOL’s whitelist allows emails to go through and be place directly into the inboxes with images displayed.

There are more filters used by AOL. However, if enhanced whitelisting status has been achieved with AOL then some or all of these following filters are bypassed. These filters include volume filters, content filters and recipient filters. If the sender domain or IP is not on AOL’s whitelist, the email is then routed through AOL's volume filters. The content filters then applies its proprietary content filtering and Bayesian filtering rules. Recipient Filters are when an email may pass a domain block, recipients personal Bayesian (defined by previous actions) rules or recipient’s personal filters. If an email does not pass these filters the email will be delivered, but then placed in the spam folder or completely blocked.

The best way to improve deliverability with AOL is to setup a Feedback Loop with them to reduce the number of complaints they receive from their end users. AOL has also planned to introduce SPF & DKIM into the reputation model soon. This means they will be using it as a means of authentication and reputation checking for connecting IPs as an added data point to help them determine the final status of email. Keeping content relevant and fresh along with emailing to active users also helps in increasing deliverability.

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