There is a free version - 10GB mailbox, 1000 seats.

https://admin.yandex.ru/select-organization?productId=free&f...

At 129 RUB(~1.50 EUR)/month, it isn't free but it's drastically cheaper than O365 or G Suite. However i wonder how often it would be blocked by useless "block all Russia and China" firewall rules.