List size is one of the strongest predictors of email response rates, including both open and click-through responses, according to a new study by email service provider ExactTarget (yes, ExactTarget advertises on this site all you transparency freaks). There appears to be an inverse relationship between list size and email responsiveness. According to the study, open and click-through rates both decrease steadily as list size increases. Exact Target claims this is indicative that list segmentation should be considered as list size grows. You can check out the entire study here.
