%% You should probably cite draft-ietf-uta-smtp-require-tls instead of this I-D.
@techreport{fenton-smtp-require-tls-01,
    number =    {draft-fenton-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-fenton-smtp-require-tls/01/},
    author =    {Jim Fenton},
    title =     {{SMTP Require TLS Option}},
    pagetotal = 10,
    year =      2016,
    month =     feb,
    day =       13,
    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 prioritized over security. This document describes a complementary SMTP service extension, REQUIRETLS. If the REQUIRETLS option is used when sending a message, it causes message delivery to fail if a TLS connection with the required security characteristics cannot be completed with the next hop MTA or if that MTA does not also advertise that it supports REQUIRETLS. Message originators may therefore expect transport security to be used for messages sent with this option.},
}