Email Subscription Bombing

[Ethanlad123]

Recently solicited an invite for a an invite-only service on 4chan by posting my primary email (I know I'm stupid) and got absolutely demolished by spam emails from otherwise legitimate email subscription mailing lists. Any idea on how this is done? I've had to manually unsubscribe from every email, ranging from Walmart to Zara. As far as I can tell, you cannot subscribe to these services without confirming the email subscription, which I'd obviously never done.