The {disarmed} flag in the subject line of a filtered email indicates the system has located a "Web Bug" or tracking code and has disabled it.

A "Web Bug" is a piece of code attached to an email that is invisible to the reader, but sends tracking information to a remote server that would then keep track of information such as when you opened the email, what kind of computer you are using, what your IP address is and so forth which the sender (or sending server) could use to identify you. This type of tracking code is most often found in HTML "newsletters" and spam messages to track when users open and read their e-mails.

The filters prevent that information from getting back to the sender to protect privacy. This helps prevent the further propagation of spam, because once an email address is verified as real, and that a human viewed it, the now-confirmed email address will tend to make its way to other senders.