Capitalization of "A" in "Dear All" [duplicate]
Solution 1:
The rule I've always followed is: When a word is used in place of a proper noun, capitalize. In this case, you are directly addressing "All," so I would capitalize: "Dear All."
Solution 2:
As when we write essay title we write this way like My Home, My Village.
Starting a letter this way is rather informal, so there are no absolute rules. I'd favour Dear All.
You might also consider things like Hello Everyone, To All Tenants, Please Note. so no problem if it starts with Dear All or Dear all.