Search is your friend; there are quite a number of posts/threads about e-mail syncing issues that go across several phones and e-mail services. Apparently the problem started a week or so ago and so far no one seems to know what the problem is or how to fix it, I'm afraid.
HAH. Fixed it myself. This is what you can do.
Settings > Accounts and services > Add account > (503 Service unavailable) > (instead of freaking out, choose OTHER ACCOUNT or Microsoft Exchange) > Enter account info. ex email@example.com and password then accept push settings. BOOOM, it works. You dont have to deal with crappy k9 email
MAKE SURE YOU UPDATED YOUR PHONE TO JELLYBEAN FIRST
I haven't been able to receive or send work e-mail (yahoo bizmail account) since December 20. I can't delete the account as I get a 503 error and I can't start over to add an account because I get a message that the account already exists. Any advice welcome to fix this for my workplace sanity!
Easiest way to access the accounts is to go into Play Store, dl the Ymail, Live/Hotmail, etc, apps, et voilà! You should now be able to access them. (I had the same issue on my Rezound, and downloading the apps corrected it easily.)