Challenges of Reputation-Based Filtering

  |  April 17, 2008   |  Comments

Reputation-based e-mail filtering is a positive development, but one that presents new challenges to individual e-mail team members.

Over the past few years, ISPs have migrated en mass from content-based filtering to reputation-based filtering. Content-based filtering involves examining e-mail copy for key words and phrases that are indicative of spam (or not spam) and scoring messages based on the results. Reputation-based filtering makes no attempt to analyze message content, but rather bases decisions on negative (complaints, bounces, protocol violations) and positive (marking as not spam, loading images, adding senders to address books) responses to messages. Such filtering is longer-term and typically based on sending IP address, though some systems use DomainKeys (define) signatures.

This change should be, and is, a positive step for legitimate e-mail marketers, but it does present some challenges for various individuals on an e-mail marketing team.

Copy Editors

The idea that there are individual words that shouldn't be used in e-mail copy is still quite pervasive. Certainly there are still some company filters that block single words (try putting "gevalia" into an e-mail sometime), but by and large single words have been passé for a very long time. Even use of phrase matching is in significant decline.

If your message is blocked or shunted to the bulk folder at a major ISP, there's little value in going over the copy with a fine-tooth comb to find the problem. It's almost certainly not there. There's also little value in testing copy variants for filtering. Testing copy variants for user response is extremely valuable, but looking for filtering effects is not.

List Managers

In essence, reputation-based filtering amounts to a heightened focus on list hygiene. Maintaining low bounce and complaint rates is more critical than ever to long-term deliverability. Make no mistake, reputation-based blocking and filtering have longer-term impacts. Whereas content filters look at individual campaigns, reputation filters monitor your entire communications stream over a significant period. In some cases this may be as long as a month. A single campaign sent to a bad list can not only get you blocked in the short term, it can also increase the likelihood of other campaigns being blocked over coming weeks. This digs a hole only good list hygiene can get you out of. Removing nondeliverables, honoring unsubscribe requests, and registering on available feedback loops (FBLs) is essential.

Systems Administrators

Your sender reputation is usually tied to your sending IP address. This means your reputation isn't portable. If you change e-mail service providers (ESPs) or, if you mail in-house via your ISP, you'll be starting afresh with no reputation. And while it used to be that no reputation was a neutral reputation, today no reputation is a bad reputation.

Many ISPs won't whitelist IP addresses until they have a sending history. The result is a Catch-22. Without a reputation you cannot be whitelisted, but without whitelisting your campaigns are more likely to be bulked, rate limited, or even blocked -- and, hence, rate you a bad reputation. This may severely restrict which campaigns you can put on a new IP, given they ROI (define) may be severely adversely affected.

The only solution is warming up new IP addresses. Start with a small proportion of your list and gradually increase the volume as your reputation develops. This process shouldn't be confused with the technique of spreading e-mail volume across a range of IP addresses to fly under the radar (a technique commonly referred to as "snowshoeing"). The aim isn't to fly under the radar; rather, it's to demonstrate solid list management practices and so develop a good reputation.

Maintain Your Good Name

Once a good reputation has been achieved, it's important to maintain it. Letting an IP address lie fallow may cause both its reputation and its whitelist status to expire. Deviation from established patterns can also negatively affect delivery, so maintaining steady and consistent volume is important.

While the switch to reputation-based filtering is extremely widespread among major ISPs, it's only just starting in the world of corporate e-mail filtering. Companies receive so much less e-mail than ISPs do that it's much harder to effectively monitor sender reputations. The result is that in the B2B (define) world, content still matters. But in both B2B and B2C (define), it matters much less than you probably think.

As spammers' techniques evolve, so do ISPs'. Evolution is upon us, and we must adapt to succeed.

Until next time,


Want more e-mail marketing information? ClickZ E-Mail Reference is an archive of all our e-mail columns, organized by topic.

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Derek Harding

Derek Harding is the CEO and founder of Innovyx Inc., a member of the Omnicom Group and the first e-mail service provider to be wholly owned by a full-service marketing agency. A British expatriate living in Seattle, WA, Derek is a technologist by background who has been working in online marketing on both sides of the Atlantic for the last 10 years.

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