Hi @jimk
When importing lists to Klaviyo from another platform that a client is migrating from, such as MailChimp, those contacts to whom you intend to send emails should have explicitly opted into marketing comms from the client.
You are confident that the contacts in the list have opted-in, but what have they actually subscribed to? "Receiving marketing from partners" or similar? So, they haven't explicitly agreed to receive emails from your client?
I think it's less likely that the contacts in such a list will be engaged or receptive to your client's emails, even if they are their target audience. Higher levels of unsubscribes/spam too. All of which will damage your client's sender reputation and brand.
I appreciate that CAN-SPAM allows unsolicited emails to be sent if there's an unsubscribe - the UK has much stricter permission-based regulations! But personally, I don't think it's good business practice, even if Klaviyo did allow it.
@Taylor Tarpley may comment further, and Klaviyo's official position is “No third-party lists“ and “only permission-based sending”, however, I hope it's helpful to have a non-Klaviyo employee comment here too.
Regards
Andy