How is the default eMail address in the eMail Subsystem Configuration page used?

How is the default eMail address in the eMail Subsystem Configuration page used?

A.
It receives all mail sent in the Send eMail step as a bcc.

B.
It is the From address for emails sent by agents using Agent E-mail.

C.
It is used if no email contact is specified in the Create eMail step.

D.
It becomes the From address in the Send eMail step if no address is specified.

Explanation: