I clicked "unsubscribe" at the bottom of an email newsletter I got tired of.
This is what the web page it brought me to said:
To ensure the privacy of the subscriber base, you must enter the eMail address that this eMail was initially sent to.
If the eMail address you provide does not match that address exactly, it will not be unsubscribed.
I can’t tell you how pleased I am that they’re looking out for my privacy with such vigilance.