Letra D
" 4.3 STARTTLS on the Submission Port
STARTTLS is a valid ESMTP extension when used on the Submission port, as defined in [RFC2476]. In fact, since the submission port is by definition not a publicly referenced SMTP server, the STARTTLS extension can be particularly useful by providing security and authentication for this service."
https://www.ietf.org/rfc/rfc3207.txt
"3. Message Submission
3.1. Submission Identification
Port 587 is reserved for email message submission as specified in this document. Messages received on this port are defined to be submissions. The protocol used is ESMTP [SMTP-MTA, ESMTP], with additional restrictions as specified here."
https://tools.ietf.org/html/rfc2476