Go to your Magix account page, log in using the wrong email address. Go to User data and select the option to Change details. Correct your email address there and save the changes.
Allow a few minutes at least for the change to propagate around the Magix system and then try again to register your product. If your initial attempt fails, wait a little longer and then try again.
Should you still be unsuccessful, you will need to email infoservice@magix.net with FULL details of both the correct and incorrect email addresses and ask that they amend your customer record, which may take a few days. hence, the suggestion to try making the amendment yourself in your account!