Is it possible to send a message to all buyers?

Everything is in the title! I’d like to ask feedback/feature from customers that bought my theme. Is there any official way via Envato?

In one word - No!

Ok. Do you have any other solution to contact all the buyers?

I’m not a themeforest author and not sure how it works over there, but maybe you could send them a message through an update of your theme?