Assume that you have a Microsoft Exchange Client Access server that establishes sessions to a different Mailbox server in an Exchange Server 2013 environment. After you apply cumulative update 9 or cumulative update 10 for Exchange Server 2013, Internet Mail Access Protocol (IMAP) clients are repeatedly prompted for authentication credentials.
This is a known issue in Exchange Server 2013.
To work around this issue, follow these steps to restart your IMAP services on the Exchange Server 2013 server:
Disable the EnableGSSAPIAndNTLMAuth parameter.
Run the Set-IMAPSettings -EnableGSSAPIAndNTLMAuth $false cmdlet.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.