​@F1238974 hey, thanks for a great question!

The short answer is, it doesn’t, however there are a few caveats.

Firstly, legal stuff - in most countries in the world (other than the US, however some states have changed their privacy laws individually), you need to have opt in before you can send marketing messages to your subscribers. Not having (provable, i.e. stored in Klaviyo) opt-in can get you in trouble long before your deliverability drops.

Secondly, technical stuff - you do not want to start blasting stuff out to any uploaded lists without first warming up your sending infrastructure. Here’s a guide on how to do that from Klaviyo. If you don’t go through this process, your deliverability will be compromised as soon as you start sending.

Thirdly, tactical stuff - non-opted in contacts are more likely to unsubscribe or mark you as spam, even if they are existing customers (maybe they’ve lapsed). The more spam reports and unsubscribes you get, the worse your deliverability will be.

In conclusion, and again, in my opinion, the upload itself won’t have a direct effect that would be any worse than securing opt in first (you won’t fail as soon as you upload these contacts), however if looked at from all angles is an overall worse solution. 

Hopefully that helps you make an informed decision on your next steps!