iCloud+ Custom Email Address - Receive Ok - Sending not available

I set up a custom email domain with iCloud+. Using Cloudflare for the DNS hosting, configured settings via the wizard, and everything looks correct and sensible, and propagation has completed.

Using the mac ecosystem, so mail on macos, mail on IOS, and testing using icloud via web browser. Everything is up to date as of posting.

Behavior is I can receive email via the `address@custom.domain` - but I can't see `address@custom.domain` as an option to send email from via any client or setting. All observed behavior is consistent between devices.

If I add `test@custom.domain` - I can receive and send email without issue.

I have removed, and re-added `address@custom.domain` - but no change in behavior.

I called in to apple, and we had no way to escelate or get a different idea of what to test. The suggestion was to create a new email and give up the old one, but I am not willing to do that. I could move email out of iCloud, but trying to not do that if possible.

Any ideas are much apreicated!! Thanks!