Charitable email campaigns receive the highest click-through rate when they have exactly 25 lines of text and three or fewer images, according to research published earlier this month by mail provider Constant Contact.

Constant Contact analysed 2.1 million emails from campaigns by both for-profit and not-for-profit organisations, and produced a report entitled How Long Should Your Email Be? New Data for Nonprofit Associations, Membership Organizations, and Religious Organizations.

The analysis found that for campaigns by profit-making organisations, emails were more successful if they contained fewer images, and that the highest click-through rates came when emails contained exactly 20 lines of text.

But it found that emails from non-profit organisations worked better when they had exactly 25 lines of text.

It found that among those non-profits, membership organisations had much more leeway in the amount of information they provided, with anything between 15 and 30 lines of text being well-read.

“It appears that non-profits generally have greater leeway in the number of images and lines of text they can put into an email when it comes to how it will impact click-through rates,” wrote Jason Fidler, a communications specialist at Constant Contact, in the report.

“Non-profit associations saw the highest click-through rates for emails with 25 lines of text. These click-through rates decreased gradually for emails that exceed 30 lines of text.”