No, it's not against the rules.

It's a proxy (albeit a poor one) for abuse, we are trying to prevent new accounts sending abusive messages.

We are open to other solutions, but we need something, because we kept having spammers sending abusive tirades to other users.

Please reach out to Spyro@programming.dev if you have any better ideas.

I believe it is instance-level, or at least that is what I saw in another thread (the instance mods can filter that)