We highly encourage you to verify that the files created by a third-party will be suitable for a production or manufacturing environment and not for personal use only.
One way to tell if your file is valid before uploading to IngramSpark is by running your file through IDPF’s free ePub Validator (http://validator.idpf.org/). Be aware, this tool only accepts files up to 10 MB.
Also, there are e-retailer specific requirements IDPF does not check. For instance, images are required to have no more than 3.2 million pixels, which is built into the IngramSpark quality check, but not addressed by IDPF. Therefore, it may be possible for the ebook to pass IDPF validation, but still get rejected by IngramSpark.