According to an ExactTarget study to be released this week, the bigger the email list, the lower the open and click-through rates, reports Internet Retailer. Lists with 100,000 or more names had an average open rate of 18.2 percent and click-though of 3.6 percent. Those with 101-1,000 names had an open rate of 42.1 percent and click-through rate of 6.8 percent.

For lists of 1,001-10,000, the rates were 33.2 percent and 5.1 percent; for 10,001-100,000, the rates were 25.8 percent and 4.5 percent.

ExactTarget reports that open rates stabilize at a 15-20 percent range when list size reaches 400,000-500,000; however, the downward trend in click-through rates is unpredictable and shows greater variation.