Occasionally I will receive a reply to an email with a normal subject line (ex. Subject: Experts Exchange) and [External] will be added to the subject line (ex. Subject: [External] Experts Exchange)
When this happens there are normally multiple people on the chain and I have seen it on reply's from within my domain so the obvious thought of an external sender got cancelled out.
I use Google Apps for business and send email to other Google Apps and Exchange users, it doesnt seem to be biased to a specific email platform. Google searches didnt turn up much. If you look at the email messages posted in this forum you will see that it happened to this person as well. Totally unrelated forum and domain, just randomly found this on google.