Learn English – Copying someone in email

Etymology Online indicates that this has been is use as "with reference to" since 1707:

"with reference to," 1707, from L. in re "in the matter of," ablative case of res "matter, thing."

Fowler does object to it, as the Etymology Online page indicates. He also indicates that it's a popularized technicality from Law, where it is used from the Latin to mean "in the matter of" or "in regard to".

So in general the usage for which you are using it is correct from a grammatical standpoint.

That said, common usage is that when "RE:" is the beginning of an email subject line it specifically relates to a reply, and using it otherwise will probably be confusing to email users, many of whom may not be familiar with the term's history or denotation.

It would likely be less confusing if you were to use it in a subject line of an email, but not at the beginning:

"My comments re: suitability of unicorns in marketing materials"

instead of

"RE: unicorns in marketing"

If you don't know the person, it is probably better to include the name. If it is a formal or business message, it would be wise to drop the "Hi" and simply use the person's title, whether it's Mr. or Ms. or Dr. (or for holders of political office, Mayor or Senator or what have you), followed by their name. I am inclined to think you don't need the "Dear", either.

Dr. Benson:

I'm writing to inquire about the possibility of an internship in your department. [Etc.]

This would be sufficient and would not cause offense through unwarranted familiarity and it would not come off as too casual.

If it were an email to someone you didn't know but the setting was casual (for example, you got their email address from a message board about a shared interest), you might use the "Hi" as in:

Hi Robert,

You don't know me, but we both frequent the Sprocket Collectors forum. I was interested in one of your posts and wanted to follow up on it.