Formal communication is on the wane in modern life, but there are still times when it's the only correct way to reinforce a connection or convey information. If you're applying for a job, looking for a recommendation, or expanding your network, err on the side of formality. Don't let the slightly archaic feel of a formal business letter tempt you into using flowery, outdated language.
When you know the recipient’s name, sign it “Sincerely yours” or just “Sincerely”, or in the US “Yours truly” is acceptable. Don’t try anything more cutesy or friendly unless a) you know the recipient quite well and are certain you can be informal without seeming disrespectful, or b) you have a “trademark” letter ending that’s part of your public persona (and the talent and popularity to pull it off). Signature and printed name: Signing your name is a minimal assurance that you stand behind what was written.