The size of a marketing email campaign and the sender's industry have a significant impact on open rates, according to a recent report from Constant Contact.

The report was based on data from an analysis of the more than 100 billion emails sent in 2014 and 2015 by more than 650,000 global Constant Contact clients. The campaigns were sent by businesses in wide range of B2B and B2C industries and nonprofit groups.

Below, key findings from the report.

Campaign Size

Campaigns sent to 35 or fewer subscribers, suggesting segmentation, have the highest open rate on average (55%), far exceeding the the overall average open rate for all emails examined (22%).

In contrast, campaigns sent to more than 7,500 subscribers average a 14% open rate.

Industry

Email campaigns sent by nonprofit groups generally have higher open rates than those of B2B and B2C organizations.

Religious organizations have the highest open rates (38%), followed by medical services (32%) and nonprofit associations (28%).

The best performing B2C industry is sports and recreation (26% open rate), and the best performing B2B industry is professional services (26%).

About the research: The report was based on data from an analysis of the more than 100 billion emails sent in 2014 and 2015 by more than 650,000 global Constant Contact clients in a range of B2B, B2C, and hybrid verticals.