Even though I am all in favor of salutations and complimentary closes, when exchanging more than a couple of mails with the same person, I feel that starting yet another reply with
Dear Mr. Smith gets somewhat worn and moreover stiff, even awkward if you like.
Hence, I was wondering whether it would be considered ill-mannered to drop the salutation after the first (second, third?) mail and jump to addressing the actual subject matter directly.
Correspondingly, if I drop the salutation, should/can I omit the valediction as well?