In general, headers are supposed to contain information relevant to routing, handing, debugging, etc. The body and key hold the message and message metadata.
There’s hardly any support for headers in SMTs. It would be great if we could filter by the value of the header. There’s already a predicate for existence of a header, how much harder can it be to check the value?
That’s a great idea. I’d suggest writing a KIP for this. KIP-991 is a recently-opened one related to SMT flexibility around headers. Given that it falls under the same theme as your idea and feels similarly scoped, it’s a good one to check out and base a new KIP on.