Dear Mr. President is a salutation of a business letter. The salutation goes at the beginning, thus that's how you got Dear.
dear your honor
Yes.
Dear Esteemed Colleagues
salutation
A comma should be placed after the salutation "Dear Professor Ortiz."
Sometimes business letters include the salutation "to whom it may concern". It is better to say "Dear" and then the addressee's name, followed by a colon.
Dear, or hello
Dear Mr./Dear Ms. followed by their surname.
Salutation in a letter is the initial greeting to your reader and should be addressed accordingly. Depending on who the letter is being addressed to, a salutation can began with Dear, Dear Sir or Madam, To Whom It May Concern, or Hello.
You would normally capitalize nouns in the salutation, Dear Residents: (but this is not a significant aspect of the letter and is often ignored). If you use a longer salutation, such as "My dear Neighbors," the word dear would not be capitalized.
Sir, Dear Sir, Dear Mr. dela Cruz.