Hi Ubaldo,
I received the following information from Kerio Connect support:
There is an "Edit As New" feature within the Kerio Connect web-client. But the sender would be changed from from the original, to a new sender - so your original email is preserved and integrity maintained. Another way that this might be achievable is if the receiver were to somehow navigate through the directory structure to their mail store and edit the contents of a message that's in their inbox.
If someone were using an email client like Apple Mail they could find the copy of the message that’s stored on their local machine and edit. This can be done for any email service, Exchange, Gmail, Connect, because the user downloads a copy of their message. This, however, shouldn’t affect the copy on the server. Its always possible to prevent people from using POP or IMAP if they’re worried about that. Additionally, there is an archiving feature built in to Connect that will capture a copy of any message as it passes through the server and before it reaches the client.