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Content filtering in Mailman

The Content filtering option controls what kind of content will be allowed to pass to the list. You can use this section to prevent any MIME types you don’t wish to allow.

If this function is enabled, Mailman will first compare all messages’ attachments against the filter_mime_types option. Any matches will be stripped from the message. Mailman will then compare what is left with the pass_mime_types option. Anything that does not match will also be stripped from the message. Any multipart attachments are checked next, and removed if they are empty. If this leaves the main message empty, it is then discarded completely.

Finally, if the convert_html_to_plaintext option is enabled, Mailman will convert the message to plain text before sending it to the list.

filter content screenshot

The filter_content option controls if Mailman will filter messages send to the mailing list.

filter mime types screenshot

The filter_mime_types option is compared to all incoming messages by Mailman. Matches are stripped from messages.

pass mime types screenshot

The pass_mime_types option does another check of the attachments. However this time anything that does not match is stripped from messages.

filter filename extensions screenshot

The filter_filename_extensions will remove attachments with extensions matching any on the list. One extension may be added per line. The example image shows the default settings. NOTE: Eden and RCI currently block some file extensions from being delivered to their users, including all of the defaults blocked by mailman. If you remove a defaulted block from your list your RCI and Eden subscribers will still not be able to receive the attachment. We recommend you host files online, such as off of an Eden or RCI public_html directory, and send your list a link to its location. This will ensure that everyone will have access to the shared file. A list of blocked extensions on Eden and RCI can be found here.

pass filename extensions screenshot
The pass_filename_extensions option works in a similar way as the filter_filename_extensions explained above, but instead of blocking it, this filter will permit only the extensions that match while stripping those that do not match.

collapse alternatives screenshot

The collapse_alternatives option will collapse MIME type multipart/alternative parts to its first subpart. This MIME type refers to messages that contain multiple variations of the same message. For example, many messages of type multipart/alternative contain both text/plain and text/html for viewing the same message in multiple formats. This allows individuals using older email clients that do not support HTML to see a text/plain message while newer email clients are able to read the HTML version.

If the subparts of the message are text/plain and text/html with text/plain being the first part then having this option set to yes will ‘collapse’ the message to text/plain and the text/html part will be discarded. Having this option set to no will preserve the multipart/alternative subparts. NOTE: If you set this option to ‘no’ you will likely want to change the convert_html_to_plaintext option to also be set ‘no’ or HTML parts of the message will still be converted to text/plain.

convert html screenshot
The convert_html_to_plaintxt option tells Mailman to strip all HTML from the message and deliver it as plain text.

filter action screenshot

The filter_action option tells Mailman what to do if filter matching above causes the message to not be sent to the list. Discard simply discards the message, Reject discards it and sends a notice to the message sender, Forward to List Owner forwards it to the owner for review, and Preserve places the message into a queue directory for the Site Administrator to view but is otherwise discarded. While the Preserve option is enabled, maintenance of a Mailman list is the responsibilities of the owners.