I know that this is a problem for more than just a few of us. I know that I can just switch to Gmail (I've used yahoo (free) for over a decade) or install K9 (I hated it by the way), but there has to be some other work around.
It's a smartphone, but this is really starting to make me crazy. If I send an email or respond to an email via my Droid, I will end up with thousands of duplicate messages in the sent folder. If I do the same tasks via my laptop, it just stores one message in the sent folder...like it's supposed to.
I'm not sure what is causing the loop on the yahoo server, but VZW says it's not their problem and others have said that the Android email app is flawed.
It's more than a minor inconvenience because, I either have to use my laptop to delete all the messages in the sent folder or go one by one on the Droid. Maybe there's an app to select all messages?