电子邮件的邮件标头中NextPart是什么意思？

[kyzon]

电子邮件的标头是这样的：
----------------------------------------------------------------
Subject: Re: Task
Date: Thu, 4 Jan 2007 16:18:39 +0800
MIME-Version: 1.0
Content-Type: multipart/mixed;
boundary="----=_NextPart_000_053F_01C74082.081309A0"
X-Priority: 3
X-MSMail-Priority: Normal
X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.3028
----------------------------------------------------------------
其中boundary="----=_NextPart_000_053F_01C74082.081309A0"几乎在每封信中都有。但是每封信的NextPart都不一样。
请问NextPart后面的串值由什么决定？
是什么意思？

[yjgx007]

Refers to RFC1521
http://www.faqs.org/rfcs/rfc1521.html

[cheer2008]

急急,,谁有email客户端源码,,,我的邮箱是[e-mail],类似FOXMAIL,,我想研究一下,,,,考虑一下能不能也做个开源共享的email客户端!