Making a macro to compact this down to a single keystroke I'll leave
for someone else, I've not yet mastered mutt macro-fu.
But the sequence
        T~N;N
will tag all New messages, then toggle the New flag on all tagged
messages. The macro might be better written to do
        T~N;N^T~A
where "^T" is control-T, to finish with an untag-all.
-Bennett
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