Unable to clear deleted iCloud emails

I have 5GB of emails stored in iCloud according to iCloud on both iOS and OSX.

There are no emails in my Inbox, Archive, or Bin on iCloud.com. The same for Spam and Sent. There are no folders. There is literally not a single email in iCloud.com.

There is no Mail app on any iOS device. The Mail app on OSX is the same as iCloud.com ie empty.

And there is no way I can see to recover deleted emails as it’s possible to do with bookmarks etc for 30 days after deletion.

There are no files in iCloud Drive and nothing in any of the bins. So the emails can’t be lurking in a mailbox archive.

So where is this mysterious 5GB of emails?