Hi there @ockanakupuje,

Most deliverability issues stem from sending to people inbox providers classify as non-engaged. These people may at one time have purchased from you or showed engagement towards your email but since then have gone dormant. We highly advise leveraging our segmentation to find your best people to send to, the ones who have the highest chance of opening your email and in return have the highest chance of buying from you. Now I attached some articles around deliverability and segmentation, that I think will benefit you extremely well. 
I wanted to send you some resources that our team has helped developed overall:
How Segmentation Drives Marketing Success
Overall Segmentation Strategies and Tips

Here are some tips to improve deliverability

• Tips for Improving Deliverability

• Spam Filters and How to Avoid Them

• Strengthen Your Sender Reputation to Alleviate Deliverability Issues

• Best Practices for Hitting the Inbox

If you're not sure where to star when it comes to List Cleaning, here's a great guide that talks about an example of Inactive Profiles and Engaged Profiles.

For deliverability reasons, we recommend sending your first campaigns to your most engaged users first for at least the first 4 weeks. As you are sending your emails from a different sending infrastructure than what you had in Mailchimp, this will ensure better deliverability in the long run, and train inbox providers to place your emails in the inbox folder.

Klaviyo imports "Mailchimp ratings" into each user profile, and you can create this data to segment customers by their engagement behavior. You can then use these segments for your campaigns over the first few weeks with Klaviyo, and progressively increase the number of profiles you send your campaigns to as you see the open rates increase. We cover this in our Mailchimp Integration article from our help center, in particular, you should have a look at the section where we explain the process in more detail.

Thanks!
Alex