Apple Mail Error [MCMailErrorDomain error 1030]
The relationship between iOS and Mac OS is the problem. iCloud includes an IMAP mail server, but Apple Mail (Mac OS) and Mail (iOS) use different names for the same folders! The iCloud InBox tends to be an easy IMAP sync but all sorts of mayhem can occur due to Draft(s)/Draft, Junk/Trash and Sent/Sent Messages folder naming.
For Apple Mail device sync service to work properly it has to be a two~way sync. Take note that iCloud is generally a one~way sync from iCloud to your devices, in that order, except for mail and perhaps other special cases.
The basic set of folders in Mac OS are Inbox, Flagged, Drafts, Sent, Junk and Trash with a temporary Outbox while sending. These cannot be changed and appear in the sidebar as default. The basic set of folders on an iOS device are Inbox, Draft, Sent and Trash. You have to actually use the iOS Junk and Drafts folders for these to appear in iOS, otherwise, they remain invisible.
Choosing where you want your messages also creates confusion if you have some emails 'On My Mac' and the rest on the iCloud and other servers. My comments here pertain solely to synced messages that are NOT in your 'On My Mac' folders.
Given that Apple Mail (Mac OS) is the gathering place for ALL emails, this is the place to look first if you have a desktop or laptop computer. The controls are under Mailbox Preferences -> Account -> Mailbox Behaviors (tab).
Assuming your servers settings are correct, you will see as options, in the Mailbox Behaviors pulldown menus, the available folders where incoming mail will automatically be directed, but given iCloud / Apple Mails two~way syncing capabilities, which folder system governs, Mac OS or iOS? The answer is BOTH plus 3rd party servers, and therein lay the problem.
If an expected folder is not visible in the Mailbox Behaviors pulldown menus for Draft(s) for example (Note this folder is called Draft in iOS), select the Draft Folder option instead for that account, and the folder will be created and visible in the sidebar. You will then see Draft and Drafts as options but what is Apple to do given the different nomenclature for two separate operating systems that are trying to stay in sync? They show them both!
Note, I am working with the latest iOS but am still using Catalina. Results may vary for those using Big Sur.
When you see the [MCMailErrorDomain error 1030], this for me always occurs when there is not a secondary location (folder) for the content to be located on the device requesting to delete the folder. Stated otherwise, you can't put a cookie into a jar if there is no jar to place the cookie, but curiously, you can place an email into a corresponding iOS Folder in Mac OS Apple Mail! You may want to see them both in other words!
I have yet to find a way to delete ANY Mail Folders / Subfolders in iOS, therefore the controls, once again for me, are exclusively through Mac OS and my Apple Desktop.
When I delete a Mac OS folder successfully, it disappears from iOS! When I need a new folder for example because there's no Drafts or Junk Folder, I MOVE an email in iOS to the Drafts or Junk Folder for the account in question and a new folder appears in Mac OS. Make sense? You have to use it to create it if a folder missing. Gotta create a jar to place the cookie...
Another trick to create a missing destination folder... is to select the parent folder in the Mailbox Behaviors Tab for the desired account as a destination, and this will create a new local folder based on the type of folder from which the email is being relocated.
Remember that shutting down iOS Mail in the middle of an email will automatically create a Draft Folder (this is a good thing) and a loss of power while doing the same in Mac OS will create an account specific Draft(s) Folder (even though there's a default Drafts Folder for your iCloud email there).
If you have 22 email accounts like me, you will note all sorts of automatically generated differences between the accounts over time. The extra synced folders for each email account will appear below the default folders in Apple Mail and it is here that you can open them to see the synced iOS folders, many of which are not desired nor intended. It is here that the [MCMailErrorDomain error 1030] always occurs for me.
Remember to 'use it to create it' when missing! The error tends to disappear when an optional folder is available...