To generate a mailfloss API key you first need to create a mailfloss account.
After that, go to Account Settings and click the API Key tab and finally click the Generate New API Key button to get a key.
API keys may be needed for 3rd party integrations that use the mailfloss API, such as Zoho CRMs mailfloss extension, which was written by a third party and is not officially supported.
Zapier is another app that requires the API key, as well as our official API.