1. Paper mail doesn't have to be the reference model to follow (or to selectively avoid following).
2. The US model for paper mail is not the only one.
3. The words "envelope", "header" and "body" do not contribute to worldwide understanding.
It is potentially dangerous to reuse terminology and concepts from a single paper based postal model -- for all the reasons stated: limitations in thinking, assumption of common context and confusion over reused terms with different semantics.
But, you have to start somewhere. In particular refering to one or more existing models is perfectly acceptable as long as we keep the warning fully in mind.
Also, any use of terminology in the end system must be fully defined to avoid any reuse inconsistency.
From a philosophical standpoint it would be a good idea to look at all of the available existing models (paper and electronic) and select those characteristics that apply, are obviously useful, that work and that have strong analogy with the target electronic model.
Specifically, what elements of the UK or African or whatever model would make sense in the electronic model?
Because we do need to provide labels for the concepts, is there an alternative to "envelope" that is better? It is an english word with quite a specific meaning. Perhaps "package"?