I noticed this too on the messages about beta testing zimlets. It looks like it is due to a "Senfer" line in the messages. For example:
Code:
From: mike@zimbra.com
Sender: mmorse@zimbra.com
To: mike@zimbra.com
Shows as:
Code:
Sent by: mmorse@zimbra.com
On behalf of: mike@zimbra.com
To: mike@zimbra.com
I know this doesn't help much as it's just an observation, but it seems to be something that Zimbra has started recently for some reason.