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.
Strike the Right Tone: Make the purpose of your letter clear through simple and targeted language, keeping the opening paragraph brief. You can start with, “I am writing in reference to…” and from there, communicate only what you need to say. The subsequent paragraphs should include information that gives your reader a full understanding of your objective(s) but avoid meandering sentences and needlessly long words. Again, keep it concise to sustain their attention.