This is something we all know is true, and something that everyone believes. But, Mailchimp has actually published numbers demonstrating just how bad old lists are.
Stats for the “Inactives” list (241,832 recipients):
Spam Complaints: 43
Open Rate: 6%
Click Rate: 2.4% (and 7,688 total clicks)
Bounces: 6,878 (2.8%)
Stats for the “Actives” list (69,642 recipients):
Spam Complaints: 3
Open Rate: 36.3%
Click Rate: 7.4% (and 6,925 total clicks)
Bounces: 128 (0.18%)
This is very clear data showing senders should think long and hard before mailing that list collected over months and years. Older lists have much, much worse stats, both in terms of delivery and in terms of recipient response, than newer lists. It also demonstrates that failing to mail people regularly does hurt delivery.