I got a new email address from Hey.com recently and I tried using it as my 'sending' email address in Convertkit (aff link).After a few broadcasts, I started to notice that my open rates took a huge nosedive. They went from 20-30% to about 5%. After digging around, I read this article by Convertkit saying how important it is to send from a custom domain.
Duh! That makes sense.
Since Hey.com doesn't have custom domains yet I paid for the pro plan at Protonmail to set an email address with a custom domain. I verified it with Convertkit by adding a TX hosting record.The next broadcast email I sent out was back up near 30% so it seemed to have done the trick. Try it out if you're not already sending from a custom domain.