14 karma · joined March 16, 2020
As a rule of thumb, one-click List-Unsubscribe with List-Unsubscribe-Post headers and a plain opt-out page (with confirmation if you risk such security solutions clicking on them, applicable only in B2B as you say) for the unsubscribe link in the email body.
These links should ideally be personalized (i.e. encode recipient’s email/account ID) so the opt-out page would not even require users to put their emails.
And please keep List-Unsubscribe via mailto as well, some clients may not support HTTPS POST.
I am pretty sure they (the pissed of recipients) have never even visited your site. Their emails had been submitted by persistent fraud groups hammering every possible input 24/7 for their scam and spam ops. I observe such behavior on our apps and sites, even those that you would assume no one is even aware of.
Cloudflare’s Turnstile will help you block 90% of such threats, and the final solution is to double-confirm the subscription - this way you can be pretty sure subscribers are there willingly and have not been put in by crooks.
Nowadays, I would even ditch the email input and force “Subscribe with Google/Apple/xyz” via Oauth to completely mitigate this broken unauthorized newsletter subscription flow.