Hi @ofrika 

So sorry to hear this. Incredibly stressful and upsetting for you!

I don't work for Klaviyo but I'll try to help with some things to consider.

It's often the case that spammers will spoof the email to look like it's being sent from your email when it is not. Or they may have hacked your email systems outside of Klaviyo. Have you contacted the ISP who hosts your email to look into this?

Have these campaigns been sent from your Klaviyo account? You mention that lots of unsubscribes have been replyto which is unusual behaviour if there's an unsubscribe link in the email - Klaviyo won't send emails without one being included - and most people will unsubscribe via that link. 

Have you checked if the replyto emails you've received are contacts that you have in Klaviyo? I ask as the spammer may have used lists they have obtained elsewhere.

If you believe your Klaviyo account has been hacked, change all user passwords immediately and, if not done so already, enable  2 factor authentication. With that enabled, even if someone obtains your Klaviyo login credentials they won't be able to gain access to your account.

Hopefully you'll get a response from Klaviyo here or via their tech support soon.

Regards

Andy