A short notice on an issue when you have deployed Exchange 2013 Cumulative Update 6 in coexistence in an Exchange 2007 environment. Exchange fellow Tony Redmond did a write-up on the issue here.
The issue prevents ActiveSync users whose mailbox reside on Exchange 2007 to authenticate properly when their requests are being proxied from Exchange 2013 CU6 to Exchange 2007. It has been identified in KB2997847. Alternatively, you direct Exchange 2007 EAS traffic directly to Exchange 2007 CAS servers when they are internet-facing and published.
Both articles provide links to request these hotfixes.
Another Exchange fellow, Jason Sherry, is keeping track of resolved and open Exchange 2013 CU6 issues here.