We have had a problem with newsletter subscriptions, in that we were being bombarded by hundreds of bot email registrations. Most of the emails were active accounts (they were probably obtained unlawfully), and we ended up seeing increased SPAM reporting from users (our newsletter email provider tracks these statistics for us).
I painstakingly went through all email subscriptions for the last 2 years and removed any of these bot registrations. I also disabled the API into our newsletter subscription provider, which would prevent any use of our old subscription system. Our websites now have a link, rather than a form, for subscribing to emails. The link will goto a separate site enabled by our newsletter provider, and it uses reCaptcha to verify human vs. bot registrations. This should keep our subscription system much cleaner in the future.
I apologize to anyone who got signed up for our newsletter without their knowledge. Please note that there is an unsubscribe link in those newsletters which will remove you from our subscription list. Or you can just email Paul and he will let me know to remove you manually.