When a user registers, he must enter a valid email address. 

However, with international domain names as well as mail servers supporting other languages, a valid email address does not need to be in English. (Since my site's users come from all over the world, it would make sense that I allow emails in languages other than just English.)

Can someone tell me how to correct that?  Thanks.

Yes, however, if I am correct, the client will tranlate all internation characters to ASCII (Punycode) using the IDNA standard.  So even if the mailservers only accept ASCII, it will still be okay to use international email address as long as the client user is using a IDNA enabled mail client. (Just like the way it works for client browsers like IE7.0) 

But bottomline, regardless of exactly how it works (cause I am not sure myself), dnn should not limit email address to english only cause internation emails are already here (even if most people are still accustomed to using english emails for now).