I have to say that I don't understand the approach. On one hand, addresses @inbox.ru are administered by Mail.Ru, the largest Russian free email host (although I have the impression that its usage is declining), so quite a few (arguably unwise) real people might be using them (I’ve actually got one that I haven’t touched in a decade). On the other, the process for getting an address @inbox.ru is identical to getting one @mail.ru and IIRC a couple of other alternative domains, but only this specific one is getting banned.
I know a bunch of sites who do that and the problem is usually that register emails get flagged by outlook and never arrive, causing a lot of support burden. Easier to then nudge people into the direction of Gmail or other providers that don't have these issues.
I've been down that road before. Blocking Outlook and Protonmail filters out 0% of legitimate users and 75% of bots. You do what you can so you're not always 1 step behind.
Do you have special access or such thing can be tracked from outside somehow? Could be a fun project to detect this kind of abusive behavior automatically