查看文档（手动替换文档编号）：
- https://www.rfc-editor.org/info/rfc821
- https://www.rfc-editor.org/rfc/rfc821
- https://www.rfc-editor.org/rfc/rfc821.html
- https://www.rfc-editor.org/rfc/rfc821.txt
- https://www.rfc-editor.org/rfc/rfc821.txt.pdf
- https://tools.ietf.org/html/rfc821 跳转到 rfc-editor
- https://datatracker.ietf.org/doc/html/rfc821
搜索：
- https://datatracker.ietf.org/doc/search?name=smtp+service+extension+for&sort=&rfcs=on
- https://www.rfc-editor.org/search/rfc_search.php
- https://www.rfc-editor.org/search/rfc_search_detail.php
状态
- Informational
- Experimental
- Best Current Practice
- Standards Track
- Historic
