Mailman automatically appends a “footer” to each message, which includes the unsubscribe instructions.
If your list has “content filtering” disabled, which allows sending attachments or sending HTML-formatted messages to the list, and the messages you send to the list aren’t plain text, the footer can appear as an attachment to the message instead of appearing within the text of it.
This happens because Mailman has no way of knowing where the visual “bottom” of a message is if it isn’t plain text.
How can I fix this?
One solution, if you’re the only person sending to the list, is to send only plain text messages (not HTML-formatted messages) from your e-mail program.
More generally, if you don’t need to send attachments or HTML-formatted messages to your list, you can turn on “content filtering” in Mailman to fix this. The content filtering strips out everything except the plain text of messages.
To do this:
1. View Mailman's "Content Filtering" screen
Go to the management page for your list, then click Content filtering.
2. Enable filtering
Click Yes next to "Should Mailman filter the content of list traffic according to the settings below?"
3. Press the "Submit Your Changes" button
Save the changes.
After doing this, Mailman footers should appear as part of the message text, instead of as attachments.