@misc{rfc7504,
    series =    {Request for Comments},
    number =    7504,
    howpublished =  {RFC 7504},
    publisher = {RFC Editor},
    doi =       {10.17487/RFC7504},
    url =       {https://www.rfc-editor.org/info/rfc7504},
    author =    {Dr. John C. Klensin},
    title =     {{SMTP 521 and 556 Reply Codes}},
    pagetotal = 7,
    year =      2015,
    month =     jun,
    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 to support the new tests and actions specified in RFC 7505. These codes are used to indicate that an Internet host does not accept incoming mail at all. This specification is not applicable when the host sometimes accepts mail but may reject particular messages, or even all messages, under specific circumstances.},
}