eli5: when unsubscribing from an email list, how does the company know which email you are subscribing from?
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Depends how you are unsubscribing.
If you are unsubscribing from your email, the link will either
1) unsubscribe you directly from clicking the email by sending a message to their server that basically says, Unsubscribe <Insert your email here> from emails
2) The link takes you to a page that one way or another saves your email (usually this is done in the background of your client) then when you click unsubscribe, it sends the same message as 1.
Tldr, the company’s server needs two things, your email and the message, unsubscribe
The link you click to unsubscribe will be specially made for you. You’ll get a link that looks something like www.example.com/unsubscribe?subscriptionid=SOMETHING. That SOMETHING will be unique identifier that, well, identifies your subscription to that particular newsletter, and will let the website know who’s trying to unsubscribe from what.
That’s also how you (can) tell who’s opened their emails. Say there’s a company logo inside the email. It’s gonna be sent to you as a URL that points to that image. To display that logo your email client will have to download the logo from that URL. Except that URL will have an identifier like the one above that’s unique to a particular email that’s been sent out. When your client goes to that URL to fetch the company logo, the server will see the identifier and know you’ve opened that email. That’s one of the reasons why modern email clients don’t automatically load content like that.
Depends on the specific system they’re using… some may simply remove you from their marketing database entirely. Other systems pass through a list ID when you click the unsubscribe link and it defaults to that selection if it provides a list of various email lists, or it just knows to remove you from that one list based on that ID.
One thing to point out here is that most companies don’t do their own email marketing. They outsource it to external companies. And as pointed out, those companies will autogenerate an Unsubscribe link for every mailing list email — this is required by law in the US. You _must_ have a one-click unsubscribe, so it can’t ask you to login to do it, and they don’t want people to just enter any ol’ email to unsubscribe.
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