E-mail inbox providers blocked an average of 19.2% of permission-based commercial e-mail during the first half of 2006, according to e-mail deliverability concern Return Path.

The figure is a slight improvement over the second half of 2005, during which ISPs blocked an average of 21% of incoming commercial e-mail, according to Return Path.

For historical perspective, non-delivery rates were an average of 15% in 2002 and peaked in 2004 at an average 22%, according to Return Path.

Internet service providers use spam complaints as the No. 1 factor in determining whether to block incoming mail from a specific sender.

A significant reason for this year

E-mail inbox providers blocked an average of 19.2% of permission-based commercial e-mail during the first half of 2006, according to e-mail deliverability concern Return Path.

The figure is a slight improvement over the second half of 2005, during which Internet service providers blocked an average of 21% of incoming commercial e-mail, according to Return Path.

For historical perspective, non-delivery rates were an average of 15% in 2002 and peaked in 2004 at an average 22%, according to Return Path.

ISPs use spam complaints as the No. 1 factor in determining whether to block incoming mail from a specific sender.

A significant reason for this year’s improvement in overall deliverability rates is that commercial e-mailers are waking up to the impact that their reputations with ISPs have on the deliverability of their e-mail, according to Heather Goff, vice president of client management and deliverability consulting for Return Path.

“ISPs are getting more sophisticated and more accurate in their protocols for filtering,” she said. “But marketers are increasingly understanding how they’re being evaluated.”

However, she added: “Though the numbers are improving, they’re still not where we want as an industry to see them. The reason is there’s still a surprisingly low awareness [among marketers] of what their reputation is.”

The ISP that blocked the highest percentage of commercial e-mail during the first half of 2006 was Excite, which Return Path said blocked an average of 50.7%. Adelphia came in at No. 2, blocking an average of 35.5%, Return Path reported. Gmail blocked 34.3%, the company said.

Compuserve reportedly blocked the lowest percentage of incoming permission-based commercial e-mail during the first half of 2006, coming in at 11.8%

USA.net came in second best at 13.2%, according to Return Path. Yahoo and AOL blocked 15.2% and 14.1%, respectively, the company reported.

Meanwhile, Hotmail and MSN blocked 22.7% and 22.4%, respectively.

Corporate spam filters improved significantly in the first half of 2006, with Brightmail filtering 14.9% of permission-based commercial e-mail and MessageLabs filtering 20%, compared to 21.5% and 30%, respectively, during the second half of 2005, Return Path reported.