%% You should probably cite rfc7444 instead of this I-D.
@techreport{zeilenga-email-seclabel-04,
    number =    {draft-zeilenga-email-seclabel-04},
    type =      {Internet-Draft},
    institution =   {Internet Engineering Task Force},
    publisher = {Internet Engineering Task Force},
    note =      {Work in Progress},
    url =       {https://datatracker.ietf.org/doc/draft-zeilenga-email-seclabel/04/},
    author =    {Alexey Melnikov and Kurt Zeilenga},
    title =     {{Security Labels in Internet Email}},
    pagetotal = 11,
    year =      2011,
    month =     sep,
    day =       2,
    abstract =  {This document describes a header field, SIO-Label, for use in Internet Mail to convey the sensitivity of the message. The SIO- Label header field which may carry a textual representation (a display marking) and/or a structural representation (a security label) of the sensitivity of the message.},
}