Mr. – for males
Mrs./ Miss/ Ms. – for females
Mrs. – married
Miss- unmarried
Ms.-  used for either married or unmarried
(Some women have a personal preference.)
Concepts of etiquette vary from place to place.  I am in the US.
If you cannot tell/find out if the person is male or female/ married or not (by checking company website/social media page, etc.),
you can…
direct the letter to the person’s title Dear Personnel Director:
You can use the full name Dear C. Jackson:
To whom it may concern: (often considered old but I still see it)
Dear Sir or Madam:  (I rarely see this: I think it is used in the UK.  Many in my area are not fond of the word madam, which I never really hear.
Mam is used but not in business letters.)
Also, I use a colon after the salutation in a business letter and a comma after the salutation in a friendly letter.
I also use a period after Mr./Mrs./Ms.