4.1.4 Do Not Support Message/Partial

A Content-Type header value of "message/partial" allows large messages to be sent in pieces and re-assembled by the client. It was originally designed to work around transmission failures during slow delivery causing the complete message to be resent from scratch, and to work around message size restrictions of implementations of protocols like SMTP. With increased bandwidth speeds and greater connectivity, the long transmission times are more a thing of the past. Continued support for this Content-Type header value allows an avenue for content that is inappropriate to reach (or leave) the e-mail client's computer. This content could include things such as "Information disclosure" of proprietary information, unsolicited commercial e-mail (spam), and computer virus attachments.

E-mail servers attempt to protect their users from inappropriate content by implementing policy applications that run as part of the algorithm. For them to work efficiently, the complete content is incorporated into one message. For this reason, servers need to prohibit sending or receiving messages with a Content-Type header value of "message/partial".