'Yours Faithfully, Yours Sincerely or Thanks' - When to use?

Its simple to remember...

Whilst you can be sincere to someone you know... you have to rely on Faith that someone you don't know will attend to your letter properly.

Think of faith as being an uncertainty, a belief etc. If you have no idea who is going to get the letter you only have faith that it will even be read!!

GB

It is incorrect to write 'to whom it may concern'.  The correct way to address a letter to somebody you don't know in England, is Dear Sir...Yours faithfully

If you are writing to someone whose name and title you do not know, use the greeting Dear Sir or Madam, and the ending Yours faithfully, signing yourself with your initials and surname.

If you are writing to a named person, address them as Dear Mr/Mrs/Miss/Ms, and end Yours sincerely, followed by your first name and surname.

If you have met them or spoken to them by phone, or otherwise feel that you have some acquaintance with them, address them by their first name and sign yourself Yours sincerely, using your first name.

SOURCE: AskOxford