Mailchimp not adding data

We get a lot of reports when people use the Mailchimp that their data is not being propagated into Mailchimp. There can be many reasons for this.

API Key is not set right This can be difficult to determine, since the API Key for Madmimi is really long, be sure you copy and paste the API key and not try to type it in.
Check the Mailchimp List Make sure you have created a list at Mailchimp and make sure that you have put that list in the Post Office stack settings. When making a list try not to use spaces, use dashes to separate words.
Confirmation Email This is probably the number one reason why your data is not being added into Mailchimp. Mailchimp sends out a confirmation email to people who subscribe. Now, if a person does not click on the link in the confirmation email, then their informaiton will not get added to your database. We also believe that if the person deletes the email and then try to sign up again, it will not send out another email. This is a mailchimp issue, and not a stack issue.

Now go forth and make your websites great!