• by Wendy Davis , Staff Writer @wendyndavis, March 16, 2006

As anti-spam technology improves, the ability of companies to measure the results of their e-mail campaigns is deteriorating. At least, that's one apparent conclusion of a study about e-mail marketing released today by ExactTarget.

The study reported that open rates seem to have dropped to 35.5 percent in the fourth quarter of last year, from 42.5 percent in the fourth quarter of 2004. But ExactTarget also reported that this apparent decrease might be the result of measurement techniques, which rely on images embedded in the e-mails.

"Image blocking technology is responsible for declining open rates," asserts the report. "When images are not displayed in HTML e-mail, the subsequent click activity also is negatively impacted."

ExactTarget also reported that click-through rates were highest on Sundays, while open rates were highest on Fridays. The study's conclusion: While there is no one best day on which to send e-mail, companies should at least consider sending messages on Fridays and Sundays.

The report was based on data from more than 2.7 billion e-mails sent by more than 4,000 organizations last year.