Yahoo's implementation of IMAP server is far from perfect. As an AT&T customer, you will be caught between Yahoo and AT&T if you encounter any problem related to IMAP servers maintained by Yahoo. Yahoo will direct you to AT&T and AT&T does not manage those servers. What surprises me is that there does not seem to be any way of getting to the actual level 3 technical support of Yahoo just to tell them to take a look at their server. I wonder what kind of business model AT&T and Yahoo have worked out that leaves the customer in this situation.
There is a problem in the Yahoo IMAP server right now that does not fetch the message bodies of some messages. This impacts reading those emails from, say, any iPhone. I have wasted time talking to AT&T and Yahoo on these. May be I should start hunting for a new ISP.