You really shouldn't use eregi, the ereg-extension has already been thrown out of PHP by default and it contains many problems, including security-related ones. Stick to preg and use proper PHP validation, go for something like
if (!filter_var($email, FILTER_VALIDATE_EMAIL) || preg_match('/@(mailinator|hotmail|trashmail)\..+$/i', $email))
throw new Flux_RegisterError('Invalid e-mail address', Flux_RegisterError::INVALID_EMAIL_ADDRESS);
For more information visit the PHP documentation on ereg and on filter_var. Keep your servers safe, folks.