I have a client with their own domain who are getting inundated with spam - hundreds of messages per day. I’d like to route their mail through a commercial spam filtering service that would have a large blacklist, but would also allow the uploading of a whitelist to ensure their customers can get through.
It’s not a spam filtering service per se, but it has very good spam filtering. The person can keep his domain, and just update the mail handling records and email clients so that Google Apps handles email, instead. Then you have an excellent gmail-like web interface when wanted, and perfectly good IMAP service for proper email clients, and even better Microsoft Exchange PIM services (Activesync) for devices that support that.