Addresses aren't merely meant for envelopes when writing formal letters -- they are also for the top of the letter itself. Starting in the upper right or left-hand corner is your name and address. This can also be part of the letterhead above. Below your address, however you choose to present it, is the name and address of the person you are sending the formal letter to. If this isn't done, it instantly won't qualify as a formal letter, even though it may seem unnecessary.
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