Accepted file formats and limits For phase 1 onboarding, the safest supported formats are: - .csv - .xlsx - .txt with one email per line Before upload Prepare the file so that: - one column clearly contains the email addresses - there are no duplicate headers inside the file - there are no blank or comment-only rows If you are still blocked Read Upload Your First List for the full preparation flow. If a file is within limits but still fails, open the support widget and include the file type and estimated row count.