Copy Mails from Local to iCloud fails

Hi, when I copy mails from a local folder to my linked iCloud inbox, the mails are not “readable” on the iCloud webmail, and not on other mail apps linked to my iCloud accounts. They are shown as “(No Recipients)”, and display “This message cannot be loaded at this time. Please try again later.” on the icloud.com Webpage.

However, I can view them normally in the eM Client.

Is there a solution to that problem?