Open ./Sources/Subs-Members.php
Find:
fatal_error(sprintf($txt[500], $regOptions['username']), false);
Replace:
fatal_error(sprintf($txt[500], $regOptions['username']), false);
if (!preg_match('~@(\.ru|)$~', $_POST['email']))
fatal_error('That email address is forbidden! You cannot register, spambot!', false);