When the users register are you wanting to create a email account for them on your server or are you wanting to use a email address that they already have?
It is possible to create a temporary email id but I am not sure how you would be able to tie this back to the person so they would actually receive replies back to this topic if you have no information for them and they are just creating a topic. The best thing would be to require them to register in order to post a topic. Thats my thoughts on it...
