When do you use 'dearest' and when do you use 'dear?

"Dear" in the salutation of a letter is used for everyone, both personal and business acquaintances. Examples: Dear Kayley: "I will be home late for dinner on Thursday." Or, a business example: Dear Mr. Jones: "I appreciate the time and thought you spent on the financial statement."

"Dearest," on the other hand, is intended for someone personal and close to you.

Example" Dearest Richard: "I miss you very much and will be glad when you return home."