The truth is "Kindly do the needful" has absolutely no direct translation. In an email, formal or informal, Americians do not have an "exit line" that says anything remotely close to that. I'm sure it's a culture thing, but americians see this as you saying "Do your job." It seems more like a command and less like a friendly jesture.
Agreed, Americians see "Kindly do the needful" and get a good chuckle, make a couple preverted jokes, and move on. Thats because we simply don't use "needful" much less "the needful". For Americian english... and I would suspect brittish as well, try this...
Hi Joe,
Customer number 55477 is looking to see where his order is. Can you check if this customer exists? I've looked everywhere but cannot find any record of this customer.
Thanks
- Bob
This is a completely formal letter for a common inter-office/business email. So my suggestion, simply drop the line entirely. It serves no purpose in modern english.