如何理解IMAP服务器返回的BODYSTRUCTURE信息？

Question

我正在使用python的IMAPClient从IMAP服务器检索电子邮件。我检索到的属性之一是BODYSTRUCTURE。但是，我找不到任何关于如何解释返回值的文档。下面是IMAP服务器返回的主体结构

16:12.679978 <  FLAGS (NotJunk $NotJunk \Seen) BODYSTRUCTURE ((("TEXT" "PLAIN" ("CHARSET" "us-ascii") NIL NIL "QUOTED-PRINTABLE" 4888 170 NIL NIL NIL)("TEXT" "HTML" ("CHARSET" "us-ascii") NIL NIL "QUOTED-PRINTABLE" 32407 479 NIL NIL NIL) "ALTERNATIVE" ("BOUNDARY" "Apple-Mail=_3AAA7CD7-3B07-406A-83CB-2C7762C3306E") NIL NIL)("APPLICATION" "PKCS7-SIGNATURE" ("NAME" "smime.p7s") NIL NIL "BASE64" 2414 NIL ("ATTACHMENT" ("FILENAME" "smime.p7s")) NIL) "SIGNED" ("BOUNDARY" "Apple-Mail=_DF4FE6BB-F796-46D7-A593-9723F4315DD2" "MICALG" "sha1" "PROTOCOL" "application/pkcs7-signature") NIL NIL))

下面是解析为python类型的相同主体结构。

"BODYSTRUCTURE": [
    [
        [
            [
                "TEXT",
                "PLAIN",
                [
                    "CHARSET",
                    "us-ascii"
                ],
                null,
                null,
                "QUOTED-PRINTABLE",
                4888,
                170,
                null,
                null,
                null
            ],
            [
                "TEXT",
                "HTML",
                [
                    "CHARSET",
                    "us-ascii"
                ],
                null,
                null,
                "QUOTED-PRINTABLE",
                32407,
                479,
                null,
                null,
                null
            ],
            "ALTERNATIVE",
            [
                "BOUNDARY",
                "Apple-Mail=_3AAA7CD7-3B07-406A-83CB-2C7762C3306E"
            ],
            null,
            null
        ],
        [
            "APPLICATION",
            "PKCS7-SIGNATURE",
            [
                "NAME",
                "smime.p7s"
            ],
            null,
            null,
            "BASE64",
            2414,
            null,
            [
                "ATTACHMENT",
                [
                    "FILENAME",
                    "smime.p7s"
                ]
            ],
            null
        ]
    ],
    "SIGNED",
    [
        "BOUNDARY",
        "Apple-Mail=_DF4FE6BB-F796-46D7-A593-9723F4315DD2",
        "MICALG",
        "sha1",
        "PROTOCOL",
        "application/pkcs7-signature"
    ],
    null,
    null
],

数组的长度似乎不是恒定的。数组的每个元素代表什么？

Answer 1

你应该读一下RFC 3501。根据正文部分的类型(文本与其他)，字段的长度可以改变。

具体阅读第73页和74页。