Not sure if it is a coincidence that I moved over to Cloudflare DNS... but mailboxes.mydomain.com does not work.
Did you set it to bypass Cloudflare (grey cloud instead of orange)? mailboxes works for me on my Cloudflare domain, and I have it set to bypass (grey).
Thanks, I setup my domains on Cloudflare rather than through the DreamHost panel. Anyway, there was no DNS entry for mailboxes on the CloudFlare for my domain. Have added and it seems to work, am now turning orange to see if it works that way too.