I still can't say for sure what happened, but let me answer some of the questions raised here.

The first was my own observation that there was an empty line ending the header. This turned out to be a mail client pasting quirk as I suspected. I was sent the original message offline and it looks fine. There is nothing wrong that I can see with the way Yahoo formats its messages.

The attachment filter does indeed allow plain text unconditionally and should not reject this message. And it does not - the content filter does ("Your posting to the SIBERNET-L list has been rejected by the content filter"). The attachment parser and filter runs before the content filter (there is no text content to filter until the message has been parsed), so the question is why the content filter objects to the message. This list happens to be on an L-Soft server (which I only realized a few minutes ago), so I was able to just look up the content filter and it only contains two rules based on the message subject, neither of which should have blocked the message. But of course it could also have been changed (the list templates have in fact been changed recently).

There is only one keyword parser in LISTSERV and indeed it stops scanning after a space, but there is an exception. If the last character in the keyword is a comma, LISTSERV ignores the space and keeps scanning because most non-IT people will type a space after a comma like in English. Likewise, if the first character in the comment is a comma, the space is ignored.

Anyway, to troubleshoot this further I need to know what was in the CONTENT_FILTER template when the message was rejected.

  Eric