Yes, that is the subject line.. I have received few real warnings ;D
I could only send test emails from Unity. Fortunately or Unfortunately I have not experienced any real hardware issue or so.  The email subject was " message from Unity "  It seems the email-subject format is fixed and only the part has four patterns; "info", "warning", "error", "critical".   Am I correct? or is there any other alert email subject? I'd like to hear from somebody who received a real (not test) alert email from Unity!  Thank you,
Unity alert email subject