%% You should probably cite rfc8689 instead of this I-D. @techreport{ietf-uta-smtp-require-tls-01, number = {draft-ietf-uta-smtp-require-tls-01}, type = {Internet-Draft}, institution = {Internet Engineering Task Force}, publisher = {Internet Engineering Task Force}, note = {Work in Progress}, url = {https://datatracker.ietf.org/doc/draft-ietf-uta-smtp-require-tls/01/}, author = {Jim Fenton}, title = {{SMTP Require TLS Option}}, pagetotal = 15, year = 2018, month = jan, day = 16, abstract = {The SMTP STARTTLS option, used in negotiating transport-level encryption of SMTP connections, is not as useful from a security standpoint as it might be because of its opportunistic nature; message delivery is, by default, prioritized over security. This document describes an SMTP service extension, REQUIRETLS, and message header field, Require-TLS. If the REQUIRETLS option or Require-TLS message header field is used when sending a message, it asserts a request on the part of the message sender to override the default negotiation of TLS, either by requiring that TLS be negotiated when the message is relayed, or by requesting that recipient-side policy mechanisms such as MTA-STS and DANE be ignored when relaying a message for which security is unimportant.}, }