I'm fine with providers imposing some default level of spam protection but as a user I'd like the option to disable it, even better if I can disable it per domain or sender. The old technique of adding the sender to your contacts doesn't work reliably anymore.

"The old technique of adding the sender to your contacts doesn't work reliably anymore."

An Outlook user first reaching out to you, and you replying and including the unique message-ID of their original envelope in your headers, doesn't affect anything either. Absurd. The whole situation is complete dog shit.