Number of hard/soft bounces divided by the number of emails sent. This is an inexact number because some systems do not report back to the sender clearly or accurately.

3 Quick And Easy Ways To Build A Profitable Opt-In List
You finally realize that you need a good opt-in list. After reading countless articles and sought expert advices and have read many success stories of people creating a small fortune with opt-in lists you finally decide to have one of your own.