Depends on why you are corresponding. " If the letter has to be to multiple recipient, I guess you would have to include all names and addresses but this could look messy if there are many. You'd have to open "Dear Sirs" or "Dear Sirs/Mesdames" sic
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sicDepends on why you are corresponding. If it were to a Board of Trustees, maybe find the name of the Chairman and address to him/her, opening with something like:
"Dear Sir, I wish to Board to consider..."
If the letter has to be to multiple recipient, I guess you would have to include all names and addresses but this could look messy if there are many.