[kh99]

You can create a new plugin using hook register_addmember_process. This will be called whne the registration form is submitted, and the email address that the user entered will be in $vbulletin->GPC['email']. If it's a bad email address you can set it to show an error message and go back to the registration page. However, I can't tell you how you'd go about checking the email address unless you have more details about that.

To set the error message you'd create an error phrase (using the Phrase Manager in the adminCP), then you'd use the phrase varname in a call to $userdata->error().