Why was a "multipart/related" message incorrectly filtered by Mailman?

We occasionally hear from customers who find that Mailman has incorrectly “filtered” (rejected) an email message. When this happens, the list administrator gets a message like this:

Subject: Content filtered message notification

The attached message matched the listname mailing list’s content filtering rules and was prevented from being forwarded on to the list membership. You are receiving the only remaining copy of the discarded message.

Why does this happen?

This is usually caused by a bug in the sender’s mail program. The mail program is incorrectly sending a message using the “multipart/related” MIME type (if you view the raw headers of the message, you’ll see the words “multipart/related” in these messages).

According to Internet mail standards, the “multipart/related” type indicates a message comprising multiple sections that only make sense when viewed together. In other words, a program sending such a message is claiming that the different parts (often a text part, an HTML part, and possibly an image) literally do not make any sense to the viewer unless all of the parts are viewed simultaneously.

If this were true, it would make sense for Mailman to filter the message. By default, Mailman won’t allow images or HTML to be sent to a list, and a “multipart/related” message means “the entire message is worthless unless you can see all the parts”. It wouldn’t make sense for Mailman to forward just the text part.

However, we’ve seen several examples of “multipart/related” messages that contain a text part that would make perfect sense when viewed alone. The sending mail program is not working correctly in these cases.

If you wish, you can get these messages to go through to your list by adding multipart/related to the “pass_mime_types” space of the “Content Filtering” section in the Mailman administrator pages (this is the default for lists created after January 9, 2015, and you can let us know if you’d like us to do this for you on older lists). However, this may allow extra images, HTML or other unwanted data to appear on your list from mail programs that have this bug.