Stefan Bauer writes:
Collapse alterantives: YES Convert html to plaintext: NO
These options probably delete any plaintext and leave HTML as it is.
Thunderbird is showing the mail "correct". Outlook does show the attachments as attachments.
Note that they're not attachments as defined by MIME, they're labeled "inline" in the Content-Disposition header. The obvious thing for the MUA to do is to display non-HTML inline parts in order vertically, then the HTML part(s), then more non-HTML inline parts, and so on. A Sufficiently Smart MUA can probably make things a bit more attractive. Outlook just doesn't bother at all. :-(
If you must cater to Outlook, the obvious way to handle it is to configure to delete HTML parts unless they are the only text part, and in that case convert HTML to plain text. IIRC that's
Collapse alternatives: YES Convert html to plaintext: YES
If you can legally do so, you can also configure the header and footer to be completely empty (not even white space). But that second option is only useful when you want to remove "ugly" attachment buttons, and can't be used when the footer text is needed.
Before you ask the obvious question: It would be possible to try to do "reasonably user-friendly" handling in Mailman by editing the HTML part instead of adding MIME parts, but I myself won't do this because it almost certainly would result in mangled, at least partially unreadable mail in some cases. In particular, if you need to have the footer displayed for legal reasons, this could make it undisplayable even by competent MUAs like Thunderbird. It would also probably be ugly. I'm not willing to deal with the likely user complaints, nor do I like the risks it would impose on list or site administrators under some laws.
This could be done as a plugin, and if somebody wants to contribute one (and preferably offer support) I would probably support adding it as a contrib feature (that is, if the code is of a certain minimal quality).
Steve