He says open rates climb when the subject lines are in the 50-character range or 80-character range. But, perhaps counterintuitively, they fall in the middle when the length is 60 or 70.

The magnetic Quist gave the keynote address Saturday at MediaPost's Email Insiders Summit conference: "Emailing People Not Lists: Using Customer Based Metrics to Drive Performance Improvement."

Research culled from 250 million messages sent over the past two years, with 660 different subject lines, has led him to believe that a 50-character subject line touting a "powerful" offer is appealing (30% off Spring Getaway flights to Florida on Delta).

And a longer 80-character-plus line describing a newsletter in enticing fashion works (Find out Secrets to Spice up your Barbecue this weekend and all Summer Long and enter to win a New Weber Grill.)

Somehow, in the 60- to-70-character middle, he says, the subject line is either too long or not long enough.

Quist has various theories, but one is that the longer the subject line, the better chance a marketer has of presenting different concepts that may appeal to different consumers and boosting open rates. So in the above example, some may be interested in the ways to improve their grilling, while others would seek the new grill, leading to higher open rates.

Quist's research--his clients include PayPal and Intercontinental Hotels in the U.S.--showing that "long subject lines work better" goes against conventional wisdom, he said.

"Our experience tended towards the belief that long subject lines work better," he said. (The longer the better goes against conventional wisdom.) A more descriptive subject line can also build goodwill with consumers, since it can provide enough info to easily either turn them on or turn them off.

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