Cited in: RFC 7504
5 RFCs mentioned in the text of RFC 7504 (SMTP 521 and 556 Reply Codes)
| RFC | Title | Status | Date |
|---|---|---|---|
| 1846 | SMTP 521 Reply Code | Experimental | September 1995 |
| 2119 | Key words for use in RFCs to Indicate Requirement Levels | Best Current Practice | March 1997 |
| 5321 | Simple Mail Transfer Protocol | Draft Standard | October 2008 |
| 5741 | RFC Streams, Headers, and Boilerplates | Informational | December 2009 |
| 7505 | A "Null MX" No Service Resource Record for Domains That Accept No Mail | Proposed Standard | June 2015 |