Gets the response code.
The response code, if present; otherwise, an empty string.
Some status responses contain a response code, which provides additional information. Response codes are delimited in the response by square bracket characters ([...]). See the IMAP4 specification for the possible response codes and their meanings.
The ResponseCode property value contains the response
code from the status response. If a response code is not present,
the property value is the empty string.
BaseStatusResponse Class | Hunny.Mail Namespace | Status responses section in the IMAP4 specification