Learn English – Official e-mail

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.

I see two possibilities: the first is that your correspondents come from one of the few social circles where Smith is a common form of address, and hence Dear Smith a common salutation. I haven't been in one such since my schooldays, but cultures vary.

Or it may be that the only information available to them is your surname, since somebody in the hierarchy is frightened of seeming sexist by providing gender. In such a case, Dear Mr, Miss, Mrs or Ms Smith would be even less friendly than the correct Dear Sir or Madam, so Dear Smith may seem a reasonable compromise.

See also How should I address someone with a known name and unknown gender?.