Thanks, but I think you're answering a different question. my problem was in retrieving attachments, not messages. The link for the attachment was in my original post.
The error message comes from the GNU server:
The requested URL /archive/html/lilypond-user/attachments/20130513/318d5cfd/attachment.png was not found on this server.
Apache/2.2.14 Server at lists.gnu.org Port 80