Do the needful is Indian English, which has been covered on ELU.
If you're only interacting with other speakers of Indian English then feel free to use it, but avoid it in any other contexts (most Americans and Brits will think it's quaint/uneducated).
In general, the "standard" form is do what[ever] is necessary, but in OP's specific context most likely nothing like that would be used anyway. If you've just asked for an email address, it goes without saying that you want the other person to do whatever is necessary to give you that information.
I may be wrong, but I have the impression that for many Indian English speakers, "Please do the needful" carries a subtext of "This problem is too complex for me to understand or resolve myself, but I have complete faith that you will be able to deal with it, because you are very skilled in such matters"
As I said, Brits and Americans wouldn't normally use any equivalent for such a trivial problem as finding someone's email address. But if the request was for something more challenging (and crucially, if it was from a manager to a more junior worker), "Do what[ever] [you think] is necessary [to solve this problem]" might be perfectly normal. The implication there is that the manager is authorising the junior to do things he might otherwise not be "permitted" to do (in effect, the junior is being temporarily "promoted" for the duration of the problem-solving).
In that context, it should be clear that (to Americans or Brits, at least) any such phrase would probably be considered offensive/cheeky if addressed to an equal in the workplace (if the person asking isn't senior enough to confer temporary authority on you, they shouldn't be speaking to you that way).
The locution "of course" is neither polite nor impolite. The tone in which it is delivered makes all the difference.
Did you like the movie about swashbuckling pirates?
--Of course, I liked it, matey! You know how much I like to talk like a pirate, arrrh!
Would you like to come with us to the concert? I know your car is in the shop.
--Of course, it's so nice of you to offer.
Do you like those cookies I baked for you?
-- Of course I like them, it's *my* recipe after all! Why wouldn't I like them?
Best Answer
People will have different opinions about rudeness. I think in some circumstances, the imperative mood can come across as rude, because it's a very direct way of asking someone to do something. It can make your question feel more like a demand than a request.
"Tell me" is a relatively direct way of asking a question, and adding "please" doesn't really soften this. As mentioned by the commenter, asking "Can you tell me" is less direct and therefore generally perceived as more polite. (There's a similar effect with "give me": "give me the answer" sounds rude, even if accompanied by "please," while "could you give me the answer" is not quite as bad).
The following strategies for avoiding the imperative mood are often used by native speakers, although not in all circumstances:
Another reason, completely separate, why people might object is because, as MorganFR mentions, this isn't useful information at all (especially if you add it at the end of all of your questions). Especially on Stack Exchange, people often prefer questioners to be brief and to the point. Adding extraneous words may be perceived as rude, since it makes the question take longer to read and process. (Yes, I know this conflicts with what I said earlier about "can you tell me" being more polite than "tell me").