如果你的邮件客户端不支持S/MIME，那么别人发给你的邮件如果是被加密或者签名过的话，你是无法看到邮件正文的，相反，会有一个比如smime.p7m这样的附件。
查看smime.p7m附件的工具，出名的有p7mViewer(http://www.cryptigo.eu/smime.p7m/ ), 该工具其实是一个类似Outlook Express的邮件客户端，解析smime.p7m附件并且在自己的界面上显示。其实，如果只是测试用或者偶尔查看smime.p7m附件的话，不用这么复杂的工具，同时这个工具只能试用30天。
下面这个工具其实就能满足需求了:
http://www.eriugena.org/code/p7mfile/
具体的介绍：
How to read an “smime.p7m”
If you read your e-mail with a client that does not understand S/MIME encryption and someone has sent you an S/MIME encrypted message then you will see an attachment named “smime.p7m” like this one .
With Gmail you could use the “Show original” option to view the full S/MIME encoded e-mail message, cut+paste it to a text editor, save as “something.eml” and open that with Outlook Express, or similar, which understands S/MIME. Of course, you do have to have the matching private key to decrypt the message!
With other e-mail clients that do not present the option to view the unaltered e-mail, or when the “smime.p7m” attachment has been forwarded, you need a way to re-format it back into a valid S/MIME message. To do that I wrote this small program. Save the attachment to disk and feed it to “p7mfile.exe” which will format it and pass it on to Outlook Express.
标签：MIME,smime,附件,mail,message,p7m From： https://blog.51cto.com/u_16125990/6623494