Writing formal emails, When I reply to the other party's email, how should I start my e-mail?
Starting the email for the first time, I'd say Dear XXX. But should I say it again when I write to them for the second time in reply to what they've said or no greetings is needed in that case? Should I simply just say hello again or something like that?
In formal writing you would just continually use "dear." See for example the correspondence between Vladimir Nabokov and Edmund Wilson, titled appropriately, Dear Bunny, Dear Volodya. In many (North American) business contexts, subsequent letters omit the greeting in favor of a more informal, chat-like style. However, "dear" is pretty neutral and almost always appropriate.
There are no settled greetings in informal correspondence. Much depends on the context and on the status of the person you are addressing (to state the obvious). Most importantly, the greetings and the sign-offs add a much-needed touch of personal flair. You can start a "hey" or a "greetings" or whatever else works for you. Pick your approach, and use it consistently.
As long as your tone is appropriate to the situation, you cannot go too wrong.
