How would you address your higher official, Sr Official, and peers when sending a Business e-mail or begin a conversation with them in the workplace?

It all depending upon your company's work environment. In a group email you can place addresses as per hierarchy means take start from the senior most and no need to make formal words, just start communication with dear colleagues (in case of mix of juniors and seniors) otherwise you can say dear sir or put the name if you address a single person, as I said before all depending upon your company's culture and person with whom you are talking.

Thanks 

I support Mr. Manzoors answer 

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