IDK about Protomail, but Fastmail of course supports these protocols. Happy customer here, as are more than a handful of HNers.

Reply to sibling: Why would anyone call an email provider a walled garden, especially if using your own domain name?!

I pay for Fastmail and I don't even use them (waiting for multi label support) just to ensure they remain an option when they support the feature I need to move away from Google.

Switching from labels to folders has been painful as part of the FastMail transition, but I'm largely glad I made the change. Now my mail is standards-compliant and I don't have to deal with all sorts of crud when I have third party mail clients that don't support Gmail's custom stuff.

I know FastMail is working on JMAP, which ideally will be a new standard that supports labels, but if it takes off, it'll be a number of years before labels is something you can assume a given mail client will support. I use platforms that rarely have official app support (aka, not iOS or Android), so wide standards compliance is a big key.

Legitimate concern.

Yes, because at least then it's obvious how they make money.

It's baffling that people are willing to pay for another walled garden.