The activity seems to have failed due to a HTTP 400 error from the
third-party API. If you are getting the same error from the third-party
API when using Postman, then the issue is most likely due to how you are
calling the API.
As long as it would be feasible to serialize the unverified user
contacts into a JSON within the payload size limits (5MB), it might
work. However, there are other details from this question that are not
mentioned here, that will also determine the feasibility of creating
such a workflow. Specifical...
External API activity will show the format of the request body JSON
format that will be generated on the Inbound Mapping tab of the
activity, under Post Body Template. This format is also shown in the
Until External API activity adds built-in support for OAuth token
retrieval, the marketer (or consultant helping the customer) will need
to refresh the OAuth token every 24 hours (or other expiry times) and
configure those in the workflow activity Execution tab where one can
specify any custom Heade...
Workflow execution logs will typically show you the various steps taken
during the execution of that workflow, including the URLs of external
API endpoints to which calls are made. Whether GET or POST method is
used during the call is based on whether there is any request body to be
sent to the API ...