I strongly doubt this can solve your problem, but you may want to try to rebuild your Outlook identity (if you didn't already). Here you find details and how-tos, which I recommend you thoroughly read:
I'm very skeptical about it, because googling around you can find plenty of users reporting your same problem (dating back to 2011) and no effective solution. I'm afraid this looks like other unsolved issues with Office for Mac 2011 still lying there. Hope I'm wrong though. I'm assuming you have updated Outlook to the latest available version (14.3.7), FWIW.
Am afraid that none of that helped. Have tried to get updates for software - but if and when you try you get a response that say's no updates available - what can you do?
This went on for about three months, tried many forums and groups, tried Apple direct with no response at all. Tried Microsoft to see if a problem with Outlook - a good forum but no solution. No response at all from anyone at Yahoo.
The only thing I could do was to create a completely new profile on the Mac, recreate the mail profile in the new profile and all the mail at Yahoo came through fine. Then had to copy across all the files etc - from the old profile to the new. A complete bore.
You would think that the likes of Apple and Microsoft - who must know that these 'help forums' exist - might monotor what s going on and maybe tried to fix some of these things that just happen - for no good reason.