Why is outgoing email edited to read Detected as Spam by MessageLabs

I see email come in from senders where it passes through Messagelabs and the subject is edited to read "Detected as Spam" before it hits my mail gateway.  Is this by design?  This is outgoing email and it's being tagged/edited before it hits me as the recipient.  I don't use MessageLabs so why does this happen?  Partial header information is added for review.