Zulip notification mails come from
email@example.com. Is there any way to make that address (or the subject) indicate the Zulip instance for which the message is sent? Currently, it is impossible to do email filtering that distinguishes emails from different zulip instances.
I don't think so. I'd throw them an issue about it. I was chatting with the Zulip folks yesterday, and I _think_ they're going to make a label or something like that to keep track of issues reported by Rust so that if someone is interested in fixing Rust's issues w/ Zulip, they can.