Mail Add Account Error - Account Already Exists
Using El Capatain. In the Mail App in iMac I cannot see an email account. It does not appear in Mail Accounts or in Preferences. When I attempt to add the email as a new Account, I receive the error "Account Already Exists". How can I add or see this email account which for some reason disappeared and I cannot see it again?
iMac, OS X Yosemite (10.10)