How does the C-Series differentiate between incoming and outgoing email?

How does the C-Series differentiate between incoming and outgoing email?

A.
By the source IP address of the SMTP client.

B.
By the MAIL FROM field of the SMTP envelope.

C.
By the type of listener on the C-Series interface, private verses public listener.

D.
By the RCPT To field in the SMTP envelope.