Like filling a leaky bucket, you need to continually add contacts to your marketing database in order to counter it’s natural, inevitable and continued decay.
You should expect lists of email addresses for marketing campaigns to decay at a rate of about 22.5% per year, or 2% per month.
Decay occurs for a number of reasons, including employee turnover. Unfortunately, there isn’t much you can do about decay, especially when it is related to events out of your control – and some list evolution is a good thing. Your list is like a garden: it should be tended to, pruned and new things planted over time.
There are steps you can take to maintain and grow your list and hopefully, you’ll be filling your bucket faster than invalid email addresses can leak out.