Hi Gert,
If you are in "conversation mode", then this is the expected behaviour because conversations span folders (I will explain in a second) and because conversations may have multiple participants. In fact we had a big discussion as to whether the column should be "from" or "participants".
If however, you are in "message mode", then when you are in the sent folder it should show "to", since messages only have a single participant (the from header). You can switch modes from the dropdown in the application panel - this is just below the Zimbra logo. (Note that there is currently a bug in "message mode" that even though the column header states "to", the information presented is the sender - which is obviously broken

We will fix this asap)
As I mentioned, conversations span folders and may have multiple participants. To see what I mean, double click on a conversation with more than one mesage in it. In the subsequent view you will see a folder column which indicates the folder in which each message resides - including messages that you sent as part of the conversation. You can move one or more of the messages to other folders (e.g. via drag and drop) and they will still be a part of the converstation (their value in the folder column will be updated). This is quite powerful, because it lets you organize your messages in anyway you want without losing the context of the conversation. Again keep in mind that the messages that you send are part of the conversation. You may initiate a conversation by being the first one to send a message. But when one or more of the recipients responds, the response will become part of the conversation.