It can be. However Yandex is a pretty big provider, it's not only used by internet marketers, but legit people too mainly in Russia, so Instagram can't really block it all together. I think the issue is either with that specific account or on Yandex's end, it happens with certain email service providers that verification emails just don't arrive, no matter what. Some sites even give you a hint before you register, that if you want the verification email to be delivered 100%, you should use Gmail, Yahoo etc.
Have you checked the spam folder though?