Sometimes expressions like this will be uttered as a hypothetical, without any underlying desire or intent to complete the act. For example, I found this post on a JustMommies Discussion Board:
lol, my DH was at the grocery store and tells me that they have one of my favorite drinks... They stopped selling it here years ago & I really miss it. I was so excited when he said they had it and I said get me some! Then he says he was just kidding. I was bummed...I should just smack him. I said that's not nice to do to a pregnant women!
Although domestic violence is a topic often taken very seriously, I don't think that's how the above comment was intended. Though the words might cross some lines, I think they were meant to be lighthearted, rather than threatening.
That said, such over-the-top remarks, like "I could just spit on him," can be construed as extremely insensitive. Even if it's only meant in jest, it's probably best to avoid talking that way.
I'm not defending the remark; there is certainly a degree of rudeness in it. That said, I'm guessing that the statement you heard was probably not meant to sound as "dishonorable" as you fear it might be.
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).
Best Answer
In American English, I have a doubt is fairly uncommon. It is more common to say the following:
Using "doubt" as verb is commonplace, as well as using the plural noun "doubts."
As @Deepak commented, I have a doubt may be a common phrase in Indian English. As he suggests, you should just use I have a question if you're speaking American English. More info here: https://english.stackexchange.com/questions/2429/can-doubt-sometimes-mean-question
To be in doubt suggests a state of mind, implying that the author is doing some thinking and is in an ongoing process of doubting something.