Extremely low email campaign open rate

Hi everyone,
We used Klaviyo for years (and Shopify Email for a few months) in the past, then last month migrated our 70k+ subscribers to Brevo (on a shared IP). After weeks of setup, we sent out our first email campaign today (essentially using the exact same setup as before: validated domain, a list with proper subscribers only - that we actually cleaned before migration).

Our campaign today got 6% open rate, that might climb up by 9-10% by the end of the week, but nowhere near our previous fairly standard 30-35-40% open rates.

After this, our account got suspended and will be permanently suspended if it happens again :thinking: despite we paid for a whole year last month.

I was just wondering what do I need to check to resolve this ridiculously low open rate, does anyone have an idea why this has happened?