The prime culprit seems to be the "once out/always out" feature of most e-mail delivery mechanisms. If an e-mail address ends up in the opt-out or bounce files, it remains there forever. As a result, your filters keep growing, and the list you mail becomes significantly smaller than your subscriber list. Another cause is the assumption by marketers that everything is working as it should. Then one day you turn around and poof! a third of your list is gone.
Here are some preventive and diagnostic measures:
Monitor your scrub rates. When reporting your results, include the subscriber list count and calculate the percentage lost prior to delivery. This is particularly important if you work with an external database, as this information is not part of the E-mail Service Provider (ESP) report.
Know your filters.
-- Get a breakdown of the reasons records are added to the filter -- unsubscribe, bounce, Spam Complaint, etc. -- and the percentage of the list attributed to each reason.
-- Monitor the breakdowns regularly to detect any irregularities and understand the size of the filter.
-- Document additions, such as uploading a new suppression file, and code them separately. As your program ages and personnel move on, it will be useful to have a history of changes made to your file.
Understand the rules. Know which business rules cause a record to end up in a filter. Sometimes the ESP has defaults that may not be optimal for your particular program.
Make it date-driven. To avoid the once out/always out trap, make sure that a recent action takes precedence over a prior action. If a consumer opts out and subsequently opts in, make sure that the action with the latest date is the one that is heeded.
Mail bounces occasionally. The rule that causes a record to be classified as undeliverable after a certain number of soft bounces within a certain period is typically your best guess or your ESP's default. Mail the records in your bounce file now and then to see whether some are deliverable. We reinstated 10,000 subscribers for a client this way.
When you consider how hard you work to get new opt-ins, it's a crime to let them slip away undetected. Look under the hood of your technology to make sure you aren't cutting off active opt-ins.