Learn English – To Whom It May Concern, what’s the best way to address a group

According to Debrett's General Rules for Writing Letters:

The sign-off depends on the salutation. As a broad rule, if you addressed the letter to 'Dear Mr Debrett' the sign off is 'Yours sincerely'. If addressed to 'Dear Sir/Madam', then 'Yours faithfully' is correct.

If you don't know the name of the person to whom you are writing, start with 'Dear Sir/Madam' and end with 'Yours faithfully'. However, I would strongly urge you to find out the name of the HR manager. Individually addressed letters are obviously more personalised and, while it won't make a huge difference, will appear better than standardised ones.

EDIT (CLARIFICATION): The only traditionally appropriate way to start a letter to an unknown person is 'Dear Sir/Madam'.

I never put old-timey snail-mail salutations on emails.

That being said, for the first email you send to someone you don't know, yes you should have a short sentence at the top explaining who you are and/or why you picked them (of all people) to contact. (eg: "I'm having a bit of trouble with product-X, and the website suggested I send a request to this address"). This is the same kind of stuff you'd start out saying if you'd phoned, after the obligatory "hello","hi" verbal handshake that we don't do in emails.

This is done because most folks these days get a lot of email of all kinds, and thus have their finger poised over the delete button on any email from a sender they don't recognize. As a courtesy to them, you should give them all the information as soon as possible to make their decision on whether to keep your email, direct it to someone else, or (horrors!) trash it.