You can use the address @test@@test.tld for registration without getting an error that it is an invalid email address.
Quite much bugs you found today, Michael!
Blah. This is actually tricky to fix as you can use international characters in email addresses now. Why make it easy when you can make it hard? Bah.
This bug has been fixed in the latest code.
Please note the latest code is not live on the site or for download. An update will be released which contains this fix.