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.
The basic structure of a formal letter: The rules might be arbitrary, but taken together they create a formula that produces clarity. When a letter follows all the rules, the reader doesn’t have to struggle to figure out where a piece of information might be, what action they should take next, or who sent it _ everything can be found in its place. Knowing the rules of letter writing is, in that sense, akin to knowing the rules of page layout or web design _ the format of the document should support and clarify the content.