Are they likely to be around for long? Are they better or equal to gmail my current provider?
I've used Yandex for years. Not for anything important or sensitive. They only get a "C" at Qualyss Labs for security. Gmail gets an "A". I'm not sure if they have implemented 2FA yet which is imperative for business.
The do have a terrific interface with some nice features. I really like being able to have an email sent at future date - this is great for reminders.
Yandex is Russian in origin and very big over there. Not going away anytime soon.