I want to connect to Constant Contact via the API, so I have my own Constant Contact account and I have created an API key to use the API. My application will allow my customer who have Constant Contact accounts to register their Constant Contact account details in my application. Then they can create an email on my application and my application will then put it to their Constant Contact account.
Can I use my API key to insert the emails in to the other Constant Contact accounts? Or will I need an API Key for each account I want to access?
You only need to have one API key for as many users as you would like to use your application. We do recommend creating a unique key for each different integration you provide. This way we can track back to the exact integration your users may report problems with to us. Outside of this type of tracking though, there are no requirements on how many integrations you can have using the same key.