When an organization maintains an email list, they must acknowledge that the information contained in the list is changing constantly. Some people unsubscribe, some emails become defunct, and people join the list. This constant shifting can work against an organization if they do not know what is happening and work with it. The rate of users leaving your list is called the "Churn Rate" and it can be very useful to know what some standard numbers are.

Churn Rate = (Subscribers who leave your list)/Total Addresses

For example, if you had 100 addresses on your list and 10 stopped receiving your email in a month, then your Churn rate for that month would be 10%. Churn can be due to constituents unsubscribing, hard bounces, and spam complaints. Basically, any time subscribers from your email list stop getting your emails. A general guideline for Churn is between 20 and 30% annually. So, an organization that has a churn rate of 70% is probably doing something drastically wrong with their email, whereas an organization with an email churn rate below 15% is doing relatively well. The churn rate could vary significantly, depending on the demographic of an organization's constituent base.

For additional resources and information, check out these:
•    http://www.e-benchmarksstudy.com/pressrelease08.html
•    http://www.lyrishq.com/content/view/77/
•    http://www.imediaconnection.com/content/19382.asp