Ok, staff has compared notes and we're on a fool's errand.
This is found across all Android, especially HTC, and effects every mail client.
Yahoo has been sweeping under the rug that they break and fix what's broken on their end every few days per a discussion with Yahoo.
Suggest either using the web interface, intercepting with gmail as documented earlier, or trying your luck.
Definitely complain to Yahoo and definitely take their lack of service into account when forming your personal email strategy moving forward.
Note - same nonsense is happening with Hotmail accounts for the same reason.