Non deleted messages in apple mail from iCloud.com mailbox account

For a while now I remarked sometimes not being able to delete specific mails from my apple iCloud.com accountmailbox within apple mail on my mac. Over time I thought ' OK have a look at it when having time'. Now looking at it, it is meanwhile about hundreds of mails which I cannot delete, to say when I delete them the no-read flag disappears but the header is stil in the mailbox list, opening them I see that there is no content (no header, no message text) just a blank window. Furthermore there is nothing within settings which gives me more control over the messages. Expected it to be something about security or privilege settings or a message setting from the sender because is not about all received mail.

A bug within Apple Mail or do I miss a setting...