Services like Proton do a good job of keeping the body of the email private, but is there anything they can do about protecting the header information?
Metadata can reveal a great deal.
Metadata can reveal a great deal.
You can reduce what an adversary sees by not including anything useful in the subject line and the only thing you can't protect is who you're speaking to and when.
Not being able to access the email body already makes full text search impossible, among other things. It is one tradeoff for using Protonmail instead of, say, Gmail.
Encrypting header data would make most operations impossible unless you download your entire inbox first.
So the providers can work to get that to 100% and mandatory to foil active attacks.