Managing iCloud Mail Server Rules Now Possible within iOS 17 Settings
Advanced settings for second iCloud Mail account disappeared?
Unable to create icloud.com mail account
New iCloud Email, "Problem Turning On Mail"
This is dumb but are you sure you're connected to wi-fi?
If so try signing out of your icloud and back in (settings>icloud>sign out)
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I haven't seen this announced or shared anywhere, and I found it very randomly, so I'll share it in case it's useful, here it is:
Now you can create and edit iCloud Mail server rules inside iOS. Just go to Settings → iCloud → iCloud Mail → iCloud Mail Rules. It's in a very weird place IMHO, and there's no way to create a rule directly from an email inside Apple Mail, which would be 100% desirable.
Also weird: there's no equivalent in MacOS settings. In MacOS there's no way to manage iCloud Mail server rules other than going to iCloud Mail via the web and managing the rules there. If you add rules in Apple Mail in MacOS, these rules are not server rules for iCloud, they are local rules that will apply to any mail account, iCloud or not. I think all this is quite confusing, and in general mail rules could make use of a makeover in Mac/iOS, but I appreciate being able to manage iCloud Mail server rules inside iOS instead of just the web.