It appears that this request was at least partially implemented in build 7719. After upgrading recently to this build, I noticed that a message from VMWare appeared with this banner:
This message contains content from external sources.
The banner is highlighted with a pale yellow background, and it appears between the "To:" address and the [Message] tab, so it is not part of the message itself. However, I have messages from multiple external sources, and this is the only one that triggered the banner.
One benefit of this approach is that it is apparently not based on altering the message, so it presumably will not break DKIM signatures. By comparison, the suggestion to add text to the subject or body will break DKIM signatures.
Can someone from SmarterTools clarify how this feature works -- what message characteristics cause it to be displayed, whether the message can be disabled and re-enabled by the system manager, and whether the system manager can control the specific wording used?
This type of tagging has become popular because many email clients have chosen to hide the FROM address from the user, a move that is unfortunate, risky, and unjustified. SmarterMail used to do this also; now it only hides the FROM address some of the time. I wish it was never. (Hovering over the Friendly Name will always display the address.)
There is a lively debate about how much "Trust indicators" like this can actually affect user behavior. This is the only journal article that I have found on the subject, but would be interested in what others can find.