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.
Similar to business or employment_related correspondence, military correspondence also has set standards for the closing (also called a “valediction”) which one should present before one’s signature. If your company conducts business with the military – or if you are applying for a job in the military – you should be aware that “Very Respectfully” (often abbreviated as “V/R”) is used in written and emailed correspondence between military members.