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.
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.