Mailster uses Google Maps to display the location of your subscribers. To display them you have to setup a Google API Key.
Google API Manager
Visit the Google API Manager where you can manage all of your API keys.
You’ll find an “Enable API” button on the top of the page which links to the Library of available APIs.
Find the “Google Static Maps API” and “enable” it. You know have to “Create credentials”
After you confirm the creation you’ll find the created API key you can use in Mailster
You can now restrict the key to a certain domain so it can’t be used on any other.
Please wait up to 5 minutes until changes take effect.