Unable to delete IMAP mail account in Yosemite

I have an IMAP mail account in the Apple Mail client that I want to delete. I first goto the Accounts preferences pane in Mail. When I click the minus icon to delete the account, I get a message about having to do it in Internet Accounts.

However, when I goto Internet Accounts, the email account isn't listed. I see all of my mail accounts except for this particular one.

How can I delete this mail account?

Thanks!