Exchange 2013, Cannot Login Using IMAP

I have a 4 Exchange server setup with Exchange 2013, 2 CAS and 2 MBX.

I had everything setup, and working fine for about 1 month. Yesterday, IMAP and POP can no longer login.

When I use the Test-IMAPConnectivity command, I get:

Test-ImapConnectivity -ClientAccessServer:2013casserver -MailboxCredential:(Get-Credential domain\user)

The specified computer isn't a Client Access server.
    + CategoryInfo          : ObjectNotFound: (:) [Test-ImapConnectivity], LocalizedException
    + FullyQualifiedErrorId : 2DF56529,Microsoft.Exchange.Monitoring.TestImapConnectivity
    + PSComputerName        : 2013casserver.domain.net

OWA and Outlook clients work fine. Does anyone know why this is occurring?

Tom Drought