"To Whom It May Concern" is an appropriate greeting for a business letter if there is no way to ascertain the name of the recipient. It is interchangeable with other generic greetings, such as "Dear Sir or Madam." Generic greetings should be followed by colons.
Generic greetings are considered to be old fashioned in today's workplace and should only be used as a last resort. It is almost always possible to research the name of the intended recipient. "Dear [First Name] [Last Name]" is always a better option. This greeting should be followed by a comma.
In a cover letter to a job application, it is never considered appropriate to use "To Whom It May Concern," as the recipient should be listed in the job posting. If the intended recipient is not listed, it would be more appropriate to avoid a salutation altogether by substituting it with a subject line.
If using "To Whom It May Concern" in a business letter is unavoidable, it is important to be aware of etiquette regarding capitalization. It is proper to capitalize each word in the phrase as follows: "To Whom It May Concern." Capitalization here serves to add importance to each word, as if replacing the recipient's proper name with the phrase.