上周碰见捕获捕网易邮箱大师邮件客户端外发的附件，文件名偶尔会有乱码的问题。但是同样的文件使用foxmail外发文件，终端捕获的附件文件名就不是乱码。这里记录一下。
网易邮箱大师
先看一下网易邮箱大师的报文（已隐去部分信息）。
Date: Thu, 13 Oct 2022 10:32:16 +0800
From: 761508152 <[email protected]>
To: =?utf-8?Q?xxxx=40outlook.com?= <[email protected]>
Message-ID: <[email protected]>
Subject: qwe123
X-Mailer: MailMasterPC/4.17.5.1008 (Win7)
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......
--=_mailmaster-634778b1_0_29_=--
文件名信息是这两条，
中文文件名：filename="=?utf-8?B?MTExMTExMTExMTExMTEx5rWL6K+V5paH5Lu2c3Nzc3Nzc2hlbGxvc3Nzc3Nz?= =?utf-8?B?c3Nzc3NzLmRvY3g=?="
非中文文件名：filename="111111111111111AABBCCDDssssssshellossssssssssss.docx"
可以看到对于非中文的文件名，报文中是直接展示的，而中文文件名，使用的的是base64的编码（Content-Transfer-Encoding: base64 此字段已经说明），对于较长的中文文件名来说，网易邮箱大师客户端在处理上选则将其分隔成了多块。而我的程序中只是将头部字符串编码格式和尾部标记给移除了导致长的文件名会出现乱码。至于文件名多长会被分割，大概数了一下算是扩展名是41个字符，长度超过41个字符便会被分割。
Foxmail
foxmail中长的中文名不会被分割：=?GB18030?B?MTExMTExMTExMTExMTExsuLK1M7EvP5zc3Nzc3NzaGVsbG9zc3Nzc3Nzc3Nzc3MxMTExMTExMTExMTExMTGy4srUzsS8/nNzc3Nzc3NoZWxsb3Nzc3Nzc3Nzc3Nzcy5kb2N4?=。