Sometime in the last 1-2 weeks, gmail has started marking all emails from yahoo.com as spam (provided the sender isn't in the receiver's contact list). As an organization that receives plenty of legitimate unsolicited mail as a matter of normal business, this is unacceptable! Worse, while I'm able to workaround this for individual accounts with a domain whitelist entry, gmail doesn't apply whitelists to Google Groups. So these emails are still flagged for moderation when sent to groups, which can delay their delivery for days.
Google, please fix your spam rules; regardless of what your algorithms are picking up on, it's not acceptable to put a blanket block on one of the most largest email services in the world, not to mention a direct competitor!