That depends on who you are writing it to. Is it a boss? An old friend? A family member? Depending on who it is addressed to is how formal or informal it should be.
mostly "with love _________" you could say "someone" on the blank if you want it to be secrethttp://www.writeletters.net/sorority-recommendation-letter.html
example of a replied farewell letter.... Re: FarewellIt is with a heavy heart that I am writing this letter to you. Today is my last day at work and I shall miss all of you. You have been great staff...
Prakash , you must know the hierarchy and way of communication.Gaurav is reporting to you , why "Sir " is used for him.As an organization we have clear understanding on everyones roles...