Comment by Angostura

I'm an iCloud+ subscriber and have moved a couple of my e-mail addresses across to use Apple's servers (about a year ago) for 'free'.

So far, it has worked consistently with no problems. The only annoyance is iyt doesn't seem that you can break multiple icloud-hosted mailboxes out into their own GUI mailboxes in the Mail client - they all get dumped into a Mailbox called 'Cloud'