Hello, All-
I thought I'd add another possible fix to some of the issues here. My mail problem started abruptly yesterday - it may have been just after having to reset my password in my itunes account - I'm not entirely sure about the order of things as it all got fairly messed up for a bit. At any rate, I have been able to send email, but cannot receive it - only on my home computer - the mail was coming through fine on my iphone and on the cloud account. I tried the Connection Doctor advice above (very helpful, as I had two accounts not connecting). Still though I was not getting my mail on the computer.
Here's what worked for me: in mail, I went to "Mailbox" in the toolbar and then chose "Synchronize" for my account. It took a bit of time, but I let it run its course, and Voila! Fixed....for now ;-)
Best of luck, and hope this is of some help. I know I have benefitted greatly from the time you've all taken to post your advice and follow up info.