I never refer to it as a tree, only as, effectively, a tiered mesh. While it's true that this breaks down with multi-hop BGP forwarding, I think it conveys enough about how the routing works for a layperson to understand that is relatively close to the truth, no? I could potentially cut down the section on Internet routing significantly, and simply mention that it is a routing fabric that does relatively greedy hop-by-hop forwarding, though I'm not sure that will be more digestable.

You're right that BGP doesn't care about hostnames, or even individual IPs, but I'm not sure that distinction is actually relevant to someone who is being exposed to this just now.

No, I disagree. IP has no multiplexing features beyond protocol multiplexing. Port numbers that allow multiplexing a single IP among multiple applications on the same host only appear in UDP/TCP. What you are referring to is specifically called HTTP multiplexing, and has little to do with TCP.

Again, technically, you are right, however there is nothing in SMTP that precludes the sender from contacting the destination SMTP server directly (well, except that spam detection systems will freak out). The fact that SMTP supports hop-by-hop forwarding, and in many cases this is the only mode that is used, does not mean that this is necessitated by the protocol. And in the interest of making the content easier to understand, I decided describing multi-hop SMTP was simply unnecessary.

I appreciate the feedback --- it's always hard to piece together relatively complex posts like this. As I mentioned elsewhere, I specifically wanted this to be a single post such that readers could follow the communication flow end-to-end. I believe this will improve reader comprehension, though of course YMMV.