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I don't know how you have to end a letter to someone you don't know. And I really need to knowEmotion: embarrassed
Comments 
Hi,

If you used tthe person's name at the start, eg Dear Mr. Smith, say 'Yours sincerely'.

If you said eg Dear Sir / Dear Madam, the convention used to be to say 'Yours faithfully'. But today, I see 'Yours sincerely' much more often than 'Yours faithfully'.

Clive
To whom it may concern
Hi,

If you search for and read the various discusions in the forum about this expression, you'll see that most people never use it anytime in their lives.

Clive