Should you protect your Bitcoin address like your credit card number? Today, I recieved this scam email:


  We have reached the biggest milestone in our history – over a million wallets.
  
  https://[removed] will celebrate with some big prizes.
  
  In October 2013, the lucky user who created the 500,000th wallet was rewarded with 10 BTC.
  
  Read more about this: http://[removed]
  
  We have discovered that you created the wallet with number 1.000.000
  
  [removed] want to reward you with 50 BTC.
  
  To prove that you are the rightful owner of the lucky wallet,
     please reply to this email and send us your mnemonic ( the long string of words generated randomly when your account was created)
  
  After verification, your wallet will be credited instantly with the 50 BTC.
  
  Thank you that you have chosen https://[removed]


I am guessing this is a scam as (a) it was sent to two separate email addresses I have, (b) the sender's email is in no way affiliated to the websites listed in the email and (c) I don't think anyone is willing to just give away 50 BTC which is close to $42,000 USD. I was wondering if I were to give them my Bitcoin address, what would happen next? Would they be able to use it like a credit card number? Or would they just be asking for more personal information?
A bitcoin address is not like a credit card number. You can safely give your bitcoin address out publicly.

What the email is asking for is something you should never give out publicly: the mnemonic that you use (e.g. if you use Electrum, they have a 12-word mnemonic code), from which you can calculate your private keys. With this, they can easily steal all bitcoins on any address that was generated using that mnemonic.

If they actually wanted to know that you are the owner of an address, they could ask you to sign a message. There's not normally much of a reason to do this, though: I don't see a scenario in which a signed message is needed in order to give you money, instead of just the address.

If they actually just wanted to send you money (as they claim), all they need is your address.