fetchyahoo-2.10.8 (not in portage) ChangeLog says: v2.10.8 06/08/07 * fix "can't find message" bug and indeed, I wasn't able to fetch mail with the latest stable (2.10.5) - fetchyahoo was giving the above mentioned errors. I copied fetchyahoo-2.10.7.ebuild to fetchyahoo-2.10.8.ebuild and it compiled and fetched my Yahoo! mails without problems. I think fetchyahoo-2.10.8 should be bumped and stabilized, but I haven't tested anything thoroughly.
Bumped to 2.10.8.