Really?
Notes is far more RFC compliant than any other MUA I use on a regular basis.
The problem is almost certainly the MIME structure of the source emails in this case. if $original_poster can post the MIME source of a sample here, we can verify this.
(hint - in Notes 6, open one of the messages that is displaying .txt "attachments" as in-line text, then use menu option View, Show, Page Source, then copy the source to the clipboard and paste it in a response to this thread)
HTH
Chris Linfoot
http://chris-linfoot.net