sun9-23.userapi.com is a subdomain under userapi.com, which is the domain used by VK (VKontakte), one of the largest social media platforms in Russia. This particular subdomain appears to serve as infrastructure — likely a content delivery or API endpoint — and returns a 403 Forbidden error to automated browsers, which is common for services that expect only certain types of requests.
If you arrived at this address through a link or redirect from within VKontakte or a VK-related service, there's no sign that you should be suspicious. The site has a valid SSL certificate, enforces secure connections, and isn't listed on any blacklists or threat databases. It doesn't handle email, which is normal for its role. The main risk here is simply that we can't inspect the actual content because of access restrictions — but that's the intended behavior, not a red flag.
For most users, the question "is sun9-23.userapi.com safe" has a straightforward answer: it's a legitimate piece of VK's backend. You don't need to treat it as a potential scam or phishing site. If you ever see it on its own, unrelated to any VK service, reconsider, but in context it's fine.