# RFC 7504: SMTP 521 and 556 Reply Codes

> request for comments publication

**Wikidata**: [Q47479767](https://www.wikidata.org/wiki/Q47479767)  
**Source**: https://4ort.xyz/entity/rfc-7504-smtp-521-and-556-reply-codes


## References

1. RFC Index
2. DBLP Dataset 2021-01-02