Question

1. Which is the correct greeting/s to send via email?

Good Morning
Good morning/Good afternoon
Good Afternoon.
2. Should the first letter of the second word be capitalized even if it is not a proper noun?
3. What is the correct way of writing "To Whom it May Concern."

Answer 1

1 and 2. The correct greetings are Good Morning and Good Afternoon. In a letter or email if you are using the greeting as a salutation then all nouns after the first one are capitalized.

3. The correct way to write this is - To Whom It May Concern. All words are capitalized.

Answer 2

According to the Purdue OWL, To Whom It May Concern should be capitalized in a business letter, but I have seen it otherwise as well. I'm at work right now, but when I get home I'm going to consult the Gregg Reference manual for a more definitive answer.

Here is a verbatim quote from the Gregg Reference Manual:

When you are preparing a letter that may be sent or shown to a number of as yet undetermined individuals, use Dear Sir or Madam (rather than To Whom It May Concern). You may also use the simplified letter style and omit the salutation.

Note the capitalization that Gregg's uses for "To Whom It May Concern", however, it is suggested not to use this.