Hello, I've got a problem. I sent yesterday some RDD from exchange to my Android wallet and they still are not in the wallet. I don't understand exactly what does it mean that after block 260799 my RDD will be sent to a non-valid of the blockchain. Does it mean that I've just lost them?
First check the balance of your wallet address in one or two block explorers which are provided in the OP.
Contact your exchange and ask them what happens to your withdrawal.
Many exchanges have one or more confirmation requirement after withdrawal is placed.
Usually after you placed the withdrawal request, the exchange will sent to you a confirmation link by email and you need to click that and confirm.
Some need to enter password after clicking the link to confirm. Some exchanges authenticate by email and sms or via google authenticator.
If you coins are send to a wrong block chain if its possible(I don't know that), you can get it back without any problem.
But above all make sure you used the correct wallet address for withdrawal.