Our automated abuse-prevention system, Omnivore, has flagged your recent import for issues that could affect the delivery of your campaigns. Your list is likely to trigger spam filters, or generate bounces and abuse complaints.
Internet service providers set strict limits on undeliverable mail and abuse complaints, which we have to abide by as an email service provider. Omnivore uses these limits to help determine when it's necessary to issue a list warning.
To send to your list again, you'll need to undo your most recent list import to remove the new addresses. After you've removed the problematic addresses, you'll be able to send to your list.