Subject is a Cardinal property that represents the subject component in the SMTP Enhanced Status Codes service extension, as described in the Internet Standards documents: 

RFC 1893 - Enhanced Mail System Status Codes  

Servers supporting the Enhanced Status Codes extension include the value as part of almost all response lines. The status code is composed of the StatusClass, Subject, and Detail values. 

These values must appear in all 2xx, 4xx, and 5xx response lines other than initial greeting and any response to HELO or EHLO. Note that 3xx responses are NOT included in this list. 

All status codes returned by the server must agree with the primary response code, that is, a 2xx response must incorporate a 2.X.X code, a 4xx response must incorporate a 4.X.X code, and a 5xx response must incorporate a 5.X.X code. 

When responses are continued across multiple lines, the same status code appears at the beginning of the text in each line of the response. 

Servers supporting this extension must attach enhanced status codes to their responses regardless of whether or not EHLO is employed by the client. 

The default value for Subject is NODETAILS, as assigned in the Create constructor.