Started seeing this this morning in Apple Mail. I have one of those exclamation point error indicators next to "Inbox", with this error message when I click on it:
There may be a problem with the mail server or network. Verify the settings for account “IMAP Account” or try again.
The server returned the error: Account exceeded bandwidth limits. (Failure).
This is in Snow Leopard. I'm using GMail IMAP, and I am way below the size quota - I've never heard of there even being a bandwidth quota.
I'm also not getting mail from the same account to the mail app on my iPhone.
EDIT - a month later I'm seeing this, and I'm thinking of just switching Mobile Me.
EDIT AGAIN - Making community wiki. I stopped seeing the problem once I updated Snow Leopard to the latest version, but since others continue to see it...
[Update] I'm seeing this problem in 10.6.4, randomly showed up. I tried all the suggested fixes with nothing.