When you're writing a business letter or sending an email message, it's important to close your letter in a professional manner. The ideal ending for a business letter conveys your thanks and respect, without eccentricity or an overly familiar tone. Although it may seem old_fashioned, most business professionals expect written correspondence – whether via a letter or an email – to be written and formatted in a conservative manner.
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.