I sent the mail next day
Still, it doesn't matter much. Bitcoin transaction mimic tangible transactions. So they are as reversible as cash or gold, the pool can't reverse the transaction and correct the address, the coins were already gone the moment they sent them.
Also, as with the delivery analogy, it's impossible for you to prove that address isn't yours. You can prove it is
yours, but not the other way round.
If MyBitcoin weren't a scam, maybe you would be able to retrieve them by asking the service provider, alas he's long gone with your money... I also have accidentally sent to my old mybitcoin address.