NO Server Unavailable. 15 on Exchange 2013

Hello,

After a SELECT "INBOX" I'm getting NO Server Unavailable. 15

SELECT "INBOX"
NO Server Unavailable. 15

Any ideas?

I've tried all sorts of adjustments to IMAP to no avail.  This used to work on our Exchange 2010 server but since migrating to 2013 we are having issues.

Thanks

Bradley