Swoosh.ApiClient.post

You're seeing just the callback post, go back to Swoosh.ApiClient module for more information.
Link to this callback

post(url, headers, body, arg4)

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Specs

post(url(), headers(), body(), Swoosh.Email.t()) ::
  {:ok, status(), headers(), body()} | {:error, term()}

Callback invoked when posting to a given URL.