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The truth is you got spammed :) They are asking for your details in order to database them and sell them to advertising companies.
Many advertising companies purchase such databases for a cost. If I can manage 2 collect 40,000 email addresses w/ postal addresses, by spamming, and I database it, I can get at least a 50k INR if I approach a marketing firm.
If u one millions, they'd call u. And the income tax department will already be there even b4 your winnings are announced :) If u see income tax department in u r home, then yeah, the mail is real. Otherwise it's fraud.