You can register the software using the old email address and password - it is not necessary to have access to the old email unless you have forgotten the password, in which case email Magix at infoservice@magix.net giving them the old email address, serial number and purchase order details if you have them.
The better option is to login to your account at magix.com with the old email address and password. Under User data, Change details you can change the email address to the one you used for your account here; also under My Products de-activate all activations for the software.
Start the program and activate/register it with the new email address.