Yep, if any account that you don't follow has sent repeated messages to you, it will go into the anti-spam filter.

This is a very typical spam we see in Nostr (random accounts messaging everyone, all the time). Once you follow them, you tell the app knows that you trust the account and thus it doesn't go to the spam filter.

In that sense, it's quite similar to the Gmail Spam folder for marketing accounts. Repeat a few times and it all ends up there.