%% You should probably cite rfc7504 instead of this I-D.
@techreport{klensin-smtp-521code-00,
    number =    {draft-klensin-smtp-521code-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-smtp-521code/00/},
    author =    {Dr. John C. Klensin},
    title =     {{SMTP 521 Reply Code}},
    pagetotal = 4,
    year =      2014,
    month =     aug,
    day =       20,
    abstract =  {This memo defines a new Simple Mail Transfer Protocol (SMTP) reply code, 521. That code may be used to indicate that an Internet host does not accept incoming mail.},
}