According to The Law Dictionary, to sign a letter on behalf of someone else, the person signing must use the designation "p.p.," which stands for per procurationem, preceding his signature. The name of the person for whom the document is being signed must be printed above or below the signature.
According to The Law Dictionary, real estate agents, investment brokers or lenders must be granted power of attorney to legally sign on behalf of someone else. Power of attorney means that the person signing the document has the legal right to sign the document for someone else. The document is legally binding.