how to prevent creation of user when email is wrong?

Hi,
i use the CreateUserWizard in order to create users. Asp.net generates automatically a password which is sent to the user by email.
The emailaddress must be like this: firstname.name@ourdomain.be . If 'firstname.name' doesn't exist in our Exchange server ('ourdomain.be'), then:
1) the user is still created
2) the message page with: 'Mailbox unavailable. The server response was: 5.1.1 User unknown' appears
E.g: if user John Wayne introduces by error Jhn.Wayne@ourdomain.be , then this user must not be created, and only a simple Alert-message must appear. 
I tried a lot of Try / Catch / Endtry but all my attempts fail: the user is
always created and the whole message page appears. I also tried with
CreateUserWizard1_SendMailError sub.1 ...