Yahoo! Groups is Yahoo's email service; it's the result of the acquisition of eGroups and some other Yahoo! stuff.
It's been stable for a long time (since the late 90s), long enough for some specialised software to be developed to do backups of it. (Not many other websites can say that.)
Yahoo Group Archiver
The Yahoo Group Archiver is a Perl script which allows an export of "the messages (without the attachments), everything from the files section and all the images from the photo section along with their hierarchy on Yahoo".
It appears that, if you get the "Couldn't get message count" error when trying to use it, the solution is to edit the yahoo2maildir.pl file and replace the bottom line
my $url = $HTTP::URI_CLASS->new($redirect, $base)->abs($base); (under the heading
sub GetJSRedirect) with
my $url = "http://groups.yahoo.com/group/$group/messages/$begin_msgid";
More frustratingly, it appears that Yahoo blocks your IP temporarily after hitting some invisible limit of data downloaded (the Archiver will continue to "download" messages for a bit, ending up with a bunch of 0-byte files, then stop completely). It's unknown if there is a solution.
Also: sometimes, some of the downloaded messages, in the middle of an otherwise normal batch, are 0 in size - almost as if Yahoo blocked your IP for a few seconds, then stopped. Watch out for these so that you can re-download them later.
Software for backups
- Yahoo Group Archiver, Sourceforge