In these cases, a large number of government addresses from different countries were subscribed through the newsletter sign up form by the use of a script/bot or an automated process. This resulted in those email addresses receiving hundreds—if not thousand—of opt-in confirmation email as well as marketing email.
Link: Spamhaus Subscription Bombing: Things You Need Know – Return Path
On Monday, August 15 2016, some email senders started noticing an increase in the amount of IP addresses being blacklisted by the anti-spam organization Spamhaus. The Spamhaus SBL listings were the result of “Abused/ misconfigured newsletter service” as the result of spammers and malicious people “subscription bombing” newsletter signup forms.