Some contractions:

>Mr. = Mister
>Mrs. = Mistress (e.g. married woman)
>Ms. = Miss (e.g. unmarried woman, but used in US more often than Mrs. since the woman marrital status may be unknown to the caller).

Dear Sir or Madam:
Dear Madam or Sir:
To Whom It May Concern:

I am writing in regard to the ... position which I found advertised at ... web site.