With respect to addressing somebody whose gender and name is unknown:
Can I use "Dear Recipient"? If yes, why is this not used more often? As in, why is this not the standard? It sounds so classy and simple and neither too formal nor too informal, and it shares none of the problems that the other two approaches have (see below).
Also, should the R be an R, or an r? The word 'recipient' is with r, but since it's a placeholder for a person, I guess we could say Recipient, just like we say Madam, and not madam?
"To whom it may concern" sounds way too formal and also it seems incorrect: I am specifically writing to somebody in particular. Why would I ever say "to whom it may concern" if I wrote to you? There's no "may" or "whom" about it, it definitely concerns you and only you, the recipient.
"Dear Sir or Madam" has several issues with it. It's clumsy, clearly. It's also biased. Why Sir before Madam? What would happen if I said "Dear Madam or Sir"? Would that be acceptable? Maybe, but then I seem to imply that I think it's more probably that the recipient is a woman (as if I couldn't be sure, so I just slipped the Sir in there at the end just in case you were a man). Another issue could be that many people, e.g. due ambiguity caused by their biological nature, aren't a Sir nor a Madam. The recipient could also be a child, in which case you'd probably prefer to say "Young Sir" and "Young Madam".