The text “{DISARMED}” is added to email subject lines by Norton Antivirus. The Norton software is running on your recipients computer (not your computer) and modifying emails as they receive them in Outlook.

The information here for Thunderbird also applies to Mailspring: Some of my emails have {Disarmed} in the subject line. The sender didn't put it there. How do I get rid of this? | Thunderbird Support Forum | Mozilla Support

Norton Antivirus adds the “DISARMED” text to the email subject line whenever the incoming email contains read receipts or link tracking. To avoid having {DISARMED} appear on your emails when sending from Mailspring to a user that runs Norton Antivirus, turn off read receipts and link tracking in the Mailspring composer by clicking the blue “eye” and “link” icons at the bottom of the window.