This all started happening the afternoon/evening of Tuesday, 3/26/2013. The stock mail app stopped receiving new e-mails from Yahoo! Mail newer than March 26th. I can still send e-mails though. People tell me they get them so I know at least that much is working. I don't even see e-mails I send to myself.
I have tried deleting and re-adding the account. I've waited 12 hours in between deleting and adding the account. I've deleted the account, removed the battery, restarted everything and added the account again. I made sure I have more than enough room on the phone's internal memory, made sure to set the sync to 15 minutes, to get 100 messages (just in case), and added the weekends to the schedule. I've even gone so far as to install Y! Mail in the hopes it might fix whatever the problem might be. I've deleted the account, shut down, removed the battery, removed the SD Card, rebooted. Added the account, still does not receive e-mails past early March 26.
What's going on?
FYI Y! Mail seems to work, so that's a fallback plan. But I like the stock app. I can set up multiple accounts with it.