Should I write “Sincerely” or “Yours truly”?

Traditionally, the sign-off Sincerely or Yours sincerely is used in an email message or letter when you are writing to someone you have interacted with before, not a complete stranger.

Yours truly is used instead when you are writing to someone you have had no previous correspondence with, especially if you greeted them as “Dear Sir or Madam.” But the difference is no longer strictly observed in US English, and you can generally use Yours truly for someone you know without any issues.