[mdaniel]

Well, "valid," yes, but "deliverable" thankfully, no

  $ dig MX example.com.

  ;; ANSWER SECTION:
  example.com.  3600 IN MX 0 .

Almost certainly the same for its .org .net and .example friends. There were some other TLDs that were designed for use in documentation, but I don't have the RFC in front of me right now to cite them

[SoftTalker]

Some of the other examples which are "technically valid but obsolete" I would consider safe to call invalid for practical purposes.

[samwho]

Yeah, I almost put a disclaimer at the end saying that while these examples are valid according to the spec, that has no bearing on whether they’re usable in practice. Email is the Wild West.