The thought is that two Subject lines is worth rejecting, an extra at
sign in the Message-ID is not.
I'm fine with that if we think implementers will find it easier to construct a
comprehensive "likely" list versus just enforcing the spec.
It's not easier but I'm with Steve here. People are not likely to
implement a spec that says that verifiers fail due to trivial syntax
errors in the message.
At this point, the only things I'm aware of that present a risk of bad
rendering are duplicate headers and l= that doesn't cover the whole
message. That list may grow in the future, but I doubt it will grow
very fast.
R's,
John
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