邮件协议中的三种情况,对应下面的三种类型

multipart/mixed可以包含附件。
multipart/related可以包含内嵌资源。
multipart/alternative 纯文本与超文本共存

1.纯文本的,只需要一块content-type块,不需要multipart块

Date: Tue, 16 Apr 2019 17:35:19 +0800
Received: from shihan2@sopans.com([]) by  via HTTP;
 Tue, 16 Apr 2019 17:35:19 +0800 (CST)
Reply-To: shihan2@sopans.com
From: <shihan2@sopans.com>
To: shihan2@sopans.com
Subject: =?GBK?B?ztLAtLLiytQ=?=
MIME-Version: 1.0
X-Priority: 3
X-MessageID: 1555407319.5028.10108
X-Originating-IP: []
X-Mailer: Sina WebMail 4.0
X-Sina-Mid: 044D8EB6F6EF3F6FAE1A32D8B0930F609000000000000002
Content-Type: text/plain; charset=GBK
Content-Transfer-Encoding: base64

aGVsbG8=

2.内容是html的要加两块content-type块内容,一块是html一块是纯文本,并且要增加一块multipart类型块

Date: Tue, 16 Apr 2019 17:36:41 +0800
Received: from shihan2@sopans.com([]) by  via HTTP;
 Tue, 16 Apr 2019 17:36:41 +0800 (CST)
Reply-To: shihan2@sopans.com
From: <shihan2@sopans.com>
To: shihan2@sopans.com
Subject: =?GBK?B?ztLAtLLiytQ=?=
MIME-Version: 1.0
X-Priority: 3
X-MessageID: 1555407401.2205.10152
X-Originating-IP: []
X-Mailer: Sina WebMail 4.0
X-Sina-Mid: 044D8EB6F6EF3F6FAE1A32D8B0930F609000000000000002
Content-Type: multipart/alternative;
         boundary="=-sinamail_alt_849bb6f96e7dc06cb99a08e3f9c84179"

--=-sinamail_alt_849bb6f96e7dc06cb99a08e3f9c84179
Content-Type: text/plain;
        charset=GBK
Content-Transfer-Encoding: base64
Content-Disposition: inline

aGVsbG8=


--=-sinamail_alt_849bb6f96e7dc06cb99a08e3f9c84179
Content-Type: text/html;
        charset=GBK
Content-Transfer-Encoding: base64
Content-Disposition: inline

PGgxPmhlbGxvPC9oMT4=


--=-sinamail_alt_849bb6f96e7dc06cb99a08e3f9c84179--

3.有附件的话,还会增加下面两种multipart类型

Date: Tue, 16 Apr 2019 17:38:47 +0800
Received: from shihan2@sopans.com([]) by  via HTTP;
 Tue, 16 Apr 2019 17:38:47 +0800 (CST)
Reply-To: shihan2@sopans.com
From: <shihan2@sopans.com>
To: shihan2@sopans.com
Subject: =?GBK?B?ztLAtLLiytQ=?=
MIME-Version: 1.0
X-Priority: 3
X-MessageID: 1555407527.4947.4232
X-Originating-IP: []
X-Mailer: Sina WebMail 4.0
X-Sina-Mid: 044D8EB6F6EF3F6FAE1A32D8B0930F609000000000000002
Content-Type: multipart/mixed;
         boundary="=-sinamail_mix_fe895d50cd0d0669bb8a7eb8c697db19"

--=-sinamail_mix_fe895d50cd0d0669bb8a7eb8c697db19
Content-Type: multipart/alternative;
         boundary="=-sinamail_alt_f50efff67f5369967ea1a6c77020a1e7"

--=-sinamail_alt_f50efff67f5369967ea1a6c77020a1e7
Content-Type: text/plain;
        charset=GBK
Content-Transfer-Encoding: base64
Content-Disposition: inline

aGVsbG8=


--=-sinamail_alt_f50efff67f5369967ea1a6c77020a1e7
Content-Type: text/html; charset=GBK
Content-Transfer-Encoding: base64
Content-Disposition: inline

PGgxPmhlbGxvPC9oMT4=


--=-sinamail_alt_f50efff67f5369967ea1a6c77020a1e7--

--=-sinamail_mix_fe895d50cd0d0669bb8a7eb8c697db19
Content-Type: application/octet-stream; name="=?GBK?B?MS5sb2c=?="
Content-Disposition: attachment; filename="=?GBK?B?MS5sb2c=?="
Content-Transfer-Encoding: base64

MXwyNTAgUElQRUxJTklORw0K


--=-sinamail_mix_fe895d50cd0d0