An email deliverability study signals growing reliance on the Sender Policy Framework (SPF) authentication method to determine whether email is legitimate and should be delivered to an inbox.

This has resulted in a shift in the list of top 10 US ISPs, according to a Lyris study of second-quarter data, reports MarketingCharts.

According to Lyris's "EmailAdvisor ISP Deliverability Report Card" for the second quarter, permission-based email messages make it to US ISP inboxes roughly 75 percent of the time:

• AIM.com led the pack with 97 percent inbox delivery, 10 percent higher than second place RoadRunner SoCal - both of which didn't make the top 10 in the first quarter.
• Rounding out the top performers - all achieving inbox delivery rates higher than 80 percent - are Verizon, USA, Compuserve, IWon, AOL, Juno, Mac and Netzero.

For marketers seeking ways to improve inbox deliverability, the most relevant finding from the current report is the appearance of SPF authentication checks in the list of the top 10 content triggers that ISPs check, according to Stefan Pollard, Director of Consulting Services at EmailLabs.

"This is the first time we've seen SPF checks start to creep into content filter tests, which means that receivers are starting to verify that a sender's SPF authentication record is accurate," said Pollard.

Failing an SPF check carries a heavy penalty - nearly 2.4 points from the current SpamAssassin test, on a Bayesian scale that identifies a message as spam when it reaches 3.0 points or higher. That's more than double the penalty for any of the other top 10 spam triggers identified - though leaving out an SPF record does not penalize the sender. (See the Direct Marketing Association's primer to learn more about authentication.)

Other findings from the 2Q07 study:

• Between the Q1 and Q2 surveys, there's been a decrease in image spam and an increase in spam that uses PDF, PowerPoint and Excel files.
• Marketers sending permission-based emails to US-based ISPs still land in the junk/bulk folder almost 16 percent of the time:

• XO Concentric far exceeds any other ISP - banishing 56 percent of invited email to the junk/bulk folder.
• Next in line are SBC Global and Bell South, both junking 30 percent of permission-based email, and Yahoo at 26 percent.
• MSN Network, GMail and Hotmail all come in at 18 percent.
• Rounding out the top ten are PeoplePC, USA and Earthlink.

• At the other end of the spectrum, AOL delivered only 1.94 percent to the junk/bulk folder.

• Marketers sending to European ISPs face even more trouble: More than 20 percent of permission-based emails were sent to the junk/bulk folder - almost three times more than the previous quarter.

About the study (pdf): From a period beginning April 1, 2007 and ending June 30, 2007, the Lyris EmailAdvisor service monitored the full delivery trajectories of 436,558 production level, permission-based email marketing messages sent from 69 businesses and nonprofit organizations to multiple accounts at 58 ISP domains in the United States, Canada, Europe, and Australia.

Messages were chosen to represent a cross-section of legitimate publishing activities, including B2B marketers, retail, travel and finance, among others.