[1] [CITE@en[RFC 2046 - Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types]] ([TIME[2015-03-22 13:14:46 +09:00]] 版) <http://tools.ietf.org/html/rfc2046#section-5.2.3.5>

[FIG(quote)[
[FIGCAPTION[
[2] [CITE@en[RFC 2046 - Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types]]
([TIME[2015-03-22 13:14:46 +09:00]] 版)
<http://tools.ietf.org/html/rfc2046#section-5.2.3.6>
]FIGCAPTION]

> Given the potential for
>           abuse, any such request messages that are constructed
>           should contain a clear indication that they were
>           generated automatically (e.g. in a Comments: header
>           field) in an attempt to resolve a MIME
>           "message/external-body" reference.
> 

]FIG]