What is the purpose of each parameter in a MailChimp unsubscribe URL?

Question

In the following URL, that unsubscribes a user from a list:

http://xxxxx.us2.list-manage.com/unsubscribe?
    u=87f537bccxx35e53a1890e0d9&
    id=40dcxx6cd6&
    e=c4aaxx1dd6&
    c=9a6xx11963

What does each parameter do?

Answer 1

Today I had to learn what each nondescript parameter means so that I could generate URLs for each email I send through Mandrill. It wasn't that easy to discover their meaning, but here are the one's I've come up with and how I found out what they are.

http://xxxxx.us2.list-manage.com/unsubscribe? u=87f537bccxx35e53a1890e0d9& id=40dcxx6cd6& e=c4aaxx1dd6& c=9a6xx11963

URL format:

• Protocol can be http or https.
• Your username comes next
• In the example, us2 is the MailChimp datacenter where your account resides. That's different for each account. (https://apidocs.mailchimp.com/api/2.0/) It's good practice to specify a DC even though the documentation says that it isn't required. Specifying it will cut down on unnecessary latency.
• list-manage[n].com can work with or without the number at the end of the domain, or it can have a 1 or a 2. Changing that doesn't appear to matter, but I think it has something to do with their load balancing.
• /unsubscribe can also be /subscribe or /profile (The latter appears to be dependent on the "e" parameter. (See below) When you don't specify it, it states, "List member profiles cannot be updated from test campaigns or archive pages" and if you specify an invalid value, you get an error page.
• u Is a unique identifier for your account. Every list on your account uses it. (See http://kb.mailchimp.com/lists/signup-forms/find-the-unsubscribe-link-for-your-list for how you can view the various URLs for your account.)
• id is your list ID
• e is the euid as documented on https://apidocs.mailchimp.com/api/2.0/lists/subscribe.php
• c I haven't seen this one yet, but my guess is that it's the campaign ID.

Also, when you wish to prefill subscribe and unsubscribe forms, you can use the following GET params.

• EMAIL Allows you to enter the subscriber's email
• MERGE1 Allows you to enter the subscriber's first name
• MERGE2 Allows you to enter the subscriber's last name