For those that are testing, they are testing a wide variety of elements including design, frequency, calls to action, and day/time sent. eROI's survey shows this breakdown for email content testing: 85.2 percent test subject lines, 54.8 percent test calls to action, 50.9 percent test designs, 49.1 percent test copy, 41.7 percent test offers, and 36.8 percent test timing of campaigns.

In a MarketingSherpa survey, Anne Holland uncovered which tests generated the highest ROI. Eighty-five percent of marketers she surveyed said subject line tests had medium to high ROI. Ninety percent said testing offers had medium to high ROI, and 95 percent said testing email design had medium to high ROI.

Since subject lines are generally the easiest element to test, it's no surprise that 85 percent of marketers eROI surveyed are doing so. However, with only 42 percent testing offers and 51 percent testing design, huge opportunities are being missed to give campaigns higher ROI. As shown, testing offers and design will achieve a greater ROI than testing subject lines.

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