Cancelling Custom Domain email with Apple Mail

What a nightmare! Thinking that I could add my custom domain address to use with my iPhone or iPad when on the road, I activated adding my "@domain.com" email domain to Apple mail via the iCloud as directed. It was then I started getting text messages from friends saying that my email was bounced back to them as undeliverable". "What did you do?" asked a friend. So, I went on iCloud and deleted the connection between my "@domain.com" thinking that the issue was solved. At least Apple gives the impression that it is solved. WRONG! I was in contact with Apple support and they said that there was no problem. WRONG! Here's the deal...When Apple asks permission to access your @domain.com (in my case, IONOS is my domain host) it turned on a "switch" at IONOS that is NOT turned off by simply deleting the Apple iCloud access. After an arduous call with IONOS support (a sweat box with no sound absorption between their agents AND a lousy VOIP connection) the IONOS agent found the "switch" and disabled it. I ain't (sic) gonna do that again!