Anything that you'd use in an informal communication is inappropriate for a business letter. This includes slang, text_speak, emojis, and anything off_color or casual. If you're used to communicating mostly with friends, family, or even co_workers you've worked with for a long time, an appropriate closing for a business letter will probably feel pretty stilted at first. Don't worry about it – your colleague or business associate won't feel that way when he or she reads your correspondence. What seems unnatural to you will feel respectful and polite to the recipient.
Letter Text: Business letter text should be clean and readable. Avoid writing your letter in one large block of text. Break your text into several concise paragraphs. These paragraphs should be aligned to the left; this allows for easier reading. When you have completed your letter, ask someone else to read it for you. Have them glance briefly at the letter. Is there too much text on the page? Is it easy to see the distinct paragraphs?