Currently Being ModeratedJan 23, 2013 5:40 PM (in response to leh344)
Yahoo's servers are terrible. Try removing the account and re-adding it. You may need to do this a number of times before you hit one of the servers that is actually working.
The problem is not your phone, it's Yahoo.
While you're waiting for them to fix it, set up a gmail or iCloud e-mail account.