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RFC 7504: SMTP 521 and 556 Reply Codes

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RFC 7504: SMTP 521 and 556 Reply Codes

Summary

RFC 7504: SMTP 521 and 556 Reply Codes is a Request for Comments[1].

Key Facts

  • RFC 7504: SMTP 521 and 556 Reply Codes authored SMTP 521 and 556 Reply Codes — author (P50): John Klensin[2].
  • RFC 7504: SMTP 521 and 556 Reply Codes's instance of is recorded as SMTP 521 and 556 Reply Codes — instance of (P31): Request for Comments[3].
  • RFC 7504: SMTP 521 and 556 Reply Codes's publisher is recorded as SMTP 521 and 556 Reply Codes — publisher (P123): Internet Engineering Task Force[4].
  • RFC 7504: SMTP 521 and 556 Reply Codes's based on is recorded as SMTP 521 and 556 Reply Codes — based on (P144): RFC 1846: SMTP 521 Reply Code[5].
  • RFC 7504: SMTP 521 and 556 Reply Codes's based on is recorded as SMTP 521 and 556 Reply Codes — based on (P144): RFC 5321: Simple Mail Transfer Protocol[6].
  • RFC 7504: SMTP 521 and 556 Reply Codes's DOI is recorded as 10.17487/RFC7504[7].
  • RFC 7504: SMTP 521 and 556 Reply Codes's language of work or name is recorded as SMTP 521 and 556 Reply Codes — language of work or name (P407): English[8].
  • RFC 7504: SMTP 521 and 556 Reply Codes's publication date is recorded as +2015-06-01T00:00:00Z[9].
  • RFC 7504: SMTP 521 and 556 Reply Codes's RfC ID is recorded as 7504[10].
  • RFC 7504: SMTP 521 and 556 Reply Codes's main subject is recorded as SMTP 521 and 556 Reply Codes — main subject (P921): Simple Mail Transfer Protocol[11].
  • RFC 7504: SMTP 521 and 556 Reply Codes's work available at URL is recorded as https://tools.ietf.org/html/rfc7504[12].
  • RFC 7504: SMTP 521 and 556 Reply Codes's work available at URL is recorded as https://tools.ietf.org/rfc/rfc7504.txt[13].
  • RFC 7504: SMTP 521 and 556 Reply Codes's work available at URL is recorded as https://www.rfc-editor.org/rfc/rfc7504.txt[14].
  • RFC 7504: SMTP 521 and 556 Reply Codes's work available at URL is recorded as https://www.rfc-editor.org/pdfrfc/rfc7504.txt.pdf[15].
  • RFC 7504: SMTP 521 and 556 Reply Codes's number of pages is recorded as {'unit': '1', 'amount': '+7'}[16].
  • RFC 7504: SMTP 521 and 556 Reply Codes's title is recorded as SMTP 521 and 556 Reply Codes[17].
  • RFC 7504: SMTP 521 and 556 Reply Codes's DBLP publication ID is recorded as journals/rfc/rfc7504[18].

Body

Designation and Status

RFC 7504: SMTP 521 and 556 Reply Codes's instance of is recorded as SMTP 521 and 556 Reply Codes — instance of (P31): Request for Comments[3].

References

Programmatic citations — every numbered marker resolves to a verifiable graph row below.

Direct Wikidata claims

  1. [3] ↑ . RFC Index. Retrieved . wikidata.org.
  2. [2] ↑ . RFC Index. Retrieved . wikidata.org.
  3. [4] ↑ . RFC Index. Retrieved . wikidata.org.
  4. [5] ↑ . RFC Index. Retrieved . wikidata.org.
  5. [6] ↑ . RFC Index. Retrieved . wikidata.org.
  6. [7] ↑ . RFC Index. Retrieved . wikidata.org.
  7. [8] ↑ . RFC Index. Retrieved . wikidata.org.
  8. [9] ↑ . RFC Index. Retrieved . wikidata.org.
  9. [10] ↑ . RFC Index. Retrieved . wikidata.org.
  10. [11] ↑ . wikidata.org.
  11. [12] ↑ . RFC Index. Retrieved . wikidata.org.
  12. [13] ↑ . RFC Index. Retrieved . wikidata.org.
  13. [14] ↑ . RFC Index. Retrieved . wikidata.org.
  14. [15] ↑ . RFC Index. Retrieved . wikidata.org.
  15. [16] ↑ . RFC Index. Retrieved . wikidata.org.
  16. [17] ↑ . RFC Index. Retrieved . wikidata.org.
  17. [18] ↑ . DBLP Dataset 2021-01-02. Retrieved . wikidata.org.

Class ancestry

  1. [1] ↑ . Wikidata. wikidata.org.

📑 Cite this page

Use these citations when quoting this entity in research, articles, AI prompts, or wherever provenance matters. We aggregate Wikidata + Wikipedia + authoritative open-data sources; the stitched, scored, cross-referenced view is what 4ort.xyz contributes.

APA 4ort.xyz Knowledge Graph. (2026). RFC 7504: SMTP 521 and 556 Reply Codes. Retrieved May 3, 2026, from https://4ort.xyz/entity/rfc-7504-smtp-521-and-556-reply-codes
MLA “RFC 7504: SMTP 521 and 556 Reply Codes.” 4ort.xyz Knowledge Graph, 4ort.xyz, 3 May. 2026, https://4ort.xyz/entity/rfc-7504-smtp-521-and-556-reply-codes.
BibTeX @misc{4ortxyz_rfc-7504-smtp-521-and-556-reply-codes_2026, author = {{4ort.xyz Knowledge Graph}}, title = {{RFC 7504: SMTP 521 and 556 Reply Codes}}, year = {2026}, url = {https://4ort.xyz/entity/rfc-7504-smtp-521-and-556-reply-codes}, note = {Accessed: 2026-05-03}}
LLM prompt According to 4ort.xyz Knowledge Graph (aggregator of Wikidata, Wikipedia, and authoritative open-data sources): RFC 7504: SMTP 521 and 556 Reply Codes — https://4ort.xyz/entity/rfc-7504-smtp-521-and-556-reply-codes (retrieved 2026-05-03)

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