%% You should probably cite rfc7504 instead of this I-D.
@techreport{klensin-smtp-521code-06,
    number =    {draft-klensin-smtp-521code-06},
    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/06/},
    author =    {Dr. John C. Klensin},
    title =     {{SMTP 521 and 556 Reply Codes}},
    pagetotal = 8,
    year =      2015,
    month =     apr,
    day =       15,
    abstract =  {This memo defines two Simple Mail Transfer Protocol (SMTP) reply codes, 521 and 556. The 521 code was originally described in an Experimental RFC in 1995 and is in wide use, but has not previously been formally incorporated into SMTP. The 556 code was created for RFC-nullMX. These codes are used to indicate that an Internet host does not accept incoming mail at all (not just under particular circumstances).},
}