A valid mail address can be made up of: a-z, A-Z, 0-9, underscores (_), and a single dot (.). And, your extra mail address must begin with a letter.

If you put a dot in your extra address, you need at least four characters after the dot. So something like yahoo.123@yahoo.com isn't available. Make sure you've got at least four after the dot!

That's it! Now go make up an address.