I love this feature and wish something like it would come to Gmail.

I can't rely on iCloud Mail anymore due to its overly aggressive silent spam filtering. Not great if you're trying to log into an account, and you can't receive the recovery emails for that account.

That's funny, as it's the same reason I moved off Gmail. Most egregious was a reply to my message ending up in spam, and the other party was someone also on Gmail

That's where the in:anywhere search is your friend. It searches all mail.

What I mean is, the mandatory spam filter was so braindead it sent a reply to my own message to spam, which is itself absurd, but even moreso because the other party was also using Gmail

You don’t have to use an iCloud account as a target for your real email address or even for your Apple account.