• by Ross Fadner , March 25, 2004

A substantial number of permission-based emails continue to be redirected to the digital dead letter office by the overly aggressive filters of Internet Service Providers (ISPs), according to Return Path's semi-annual "Email Blocking and Filtering Report." In fact, the report by the provider of email performance monitoring tools indicates that the overall number of messages denied entry into users' inboxes rose slightly from July-December 2003.

Nearly 20 percent of opt-in commercial email sent during the second half of 2003 did not reach its intended destination, marking a 1.7 percent increase during the past six months, and a 3.7 increase over the second half of 2002. The report shows that among the top 18 ISPs, NetZero and Yahoo!'s SBC Global ISP product were the worst offenders, blocking 37.7 percent and 26.7 percent of permission-based emails, respectively. EarthLink and Bell South had the lowest non-delivery rates, at 7 percent and 8 percent, respectively. Still, more than half of the top ISPs blocked over 20 percent of legitimate commercial messages, according to Return Path.

Why the high numbers? For each ISP, the spam detection process is different.

According to the report, blacklists and other Internet Protocol address-based mechanisms determine how and why email gets blocked. Unknown user thresholds, in which an ISP blocks email from any sender whose bulk list contains too many unknown user domain names, is another factor. Other methods include content filters, which most servers have, and spam-traps, where ISPs create fake names to trap spammers who harvest or "scrape" names by glossing over Web pages in an effort to add names to their lists.

The high blocking rate of commercial email and its continued growth is a troubling prospect for permission-based email marketers. Using data from its client base, Return Path found that the marketing industry has the highest delivery failure rate among all permission-based verticals, at 50 percent.

Currently, email marketers find themselves in a sticky situation because they have no way of knowing whether an ISP blocks or re-routes the emails they send; ISPs are not required to inform either senders or recipients that messages have been blocked. This puts the practice of email metrics into question, as marketers don't really know how many of their emails were actually received.

Another problem facing marketers involves delivery rules. Procedures employed by the ISPs change frequently, and often without notification. In the Return Path report, Yahoo! had the biggest drop in deliverability compared to the first half of 2003, blocking 16 percent more permission-based email in the second half, while the SBC Global/Yahoo! ISP content package clocked 14 percent more.

Return Path attributes Yahoo!'s abrupt drop in delivery to the volume of user complaints it received about spam. Stephanie Miller, vice president, strategic services at Return Path, says: "Each ISP is making its decision (to block email) based on what they think is best for their subscribers. Remember, [blocking spam] is part of their value proposition," she says, adding: "The onus is on marketers to understand the problem and remedy it."

Neither Yahoo! or NetZero responded to interview requests by press time.