Managing list growth is a constant challenge to online merchants that communicate with customers via email; such a challenge, that it topped this year's list of a study by JupiterResearch. More than a third, of the merchants surveyed said list turnover was their biggest headache.

Its ranking surprised Elaine O'Gorman, VP of strategy at Silverpop Systems Inc., an email marketing provider that commissioned the study. "Growing a strong database of customers and prospects is obviously the core part of any strong email marketing program," she said. "All elements of sophisticated email marketing; segmentation, personalization, dynamic content, lifestyle marketing and more, depend upon the quantity and quality of the email list."

In last year's survey, marketers said analyzing email campaign results was their most difficult task. While 24 percent of respondents said they plan to deploy a viral marketing campaign, just 10 percent said such actions were, "very successful."

The study also found fewer marketers challenged by analyzing campaign results, with just one in four saying it's difficult, compared to 32 percent who said so a year ago.

"Reporting and analytic features of email marketing systems have become much more sophisticated over the last few years," O'Gorman said. "Marketers are now able to gain valuable insights into what is working, and can use that information to improve future campaigns," she added.