Understanding the benchmarks for email performance is something most people leave to whatever their neighbor told them. However, knowing what the standards are for your particular industry can be helpful in keeping a realistic expectation, or knowing when something has gone wrong.
My favorite email service provider, Mailchimp, provides a number of helpful resources and research on these benchmarks to help you compare your results with what is happening with your competition. The data that Mailchimp has collected is not only fascinating but really valuable, because 70% of their customers are small businesses with one to ten employees, and the data can be further narrowed by industry. So you are not comparing the results of large corporations who have been able to send tens of thousands of emails and cross test every word and image.