%% You should probably cite draft-klensin-smtp-521code instead of this I-D.
@techreport{klensin-rfc1846bis-00,
    number =    {draft-klensin-rfc1846bis-00},
    type =      {Internet-Draft},
    institution =   {Internet Engineering Task Force},
    publisher = {Internet Engineering Task Force},
    note =      {Work in Progress},
    url =       {https://datatracker.ietf.org/doc/draft-klensin-rfc1846bis/00/},
    author =    {Dr. John C. Klensin},
    title =     {{SMTP 521 Reply Code}},
    pagetotal = 7,
    year =      2014,
    month =     aug,
    day =       11,
    abstract =  {This memo defines a new Simple Mail Transfer Protocol (SMTP) reply code, 521, which one may use to indicate that an Internet host does not accept incoming mail. It is a standards track replacement for the earlier, experimental, RFC 1846 without any substantive changes.},
}