For corporate email, Apple's email domain in apple.com. They don't appear to use localized email domains like apple.co.uk or apple.fr. They do use sub-domains under apple.com (like support.apple.com) for various services, including email. But they have a wildcard MX record for all sub-domains, so it's not easy to tell what's valid or not. That might mean they accept misdirected mail to sub-domains and use it to feed their spam filtering (spamtraps).