What words should be capitalized in "to whom it may concern"?

"To whom it may concern" is correct according to Gregg, the only style guide I could find that addressed this issue completely. However, I found the Chicago Manual of Style using a version that capitalized every word - but they had no citation or Q&A entry to back it up. I found this web resource that backs it up, though. Purdue's OWL concurs, capitalizing it the same way.

From this page that references the Gregg Reference Manual that asks the same question as was asked here:

The first word, all nouns, and all titles are capitalized in the salutation. That's according to The Gregg Reference Manual. Gregg says nothing about the last word. As pronouns, all and everyone would not be capitalized unless they were the first word or part of someone's title, according to Gregg.

The only words that are capitalized on their own in a salutation are the first word or any proper nouns, and standing in for a name doesn't promote something to proper-noun status. Otherwise we'd have to capitalize pronouns ("I heard that He said to do this"), which is generally only done when referring to a deity.

An exception to all of this is when something is a title. Unless you're writing a book titled "To Whom It May Concern", in a letter, this should be capitalized like a sentence.

Again, please note that this is an issue of style, and there probably is no "correct" answer, but there doubtless is a standard in general usage.

I see no particular reason to put the first letter of any of the words in capitals except the first one, but it's really a matter of personal style. You could capitalise all the initial letters, or you could put the whole thing in capitals and underline it if you thought doing so served some purpose.

(You might want to consider whether you need to use such a salutation at all.)