I have a couple of users where I work that were experiencing an issue with our Exchange server not syncing their email.
Those affected where getting a “Waiting for Sync” message in their email client on their phones.
All of the Exchange settings were right, internet access was working.  Nothing had changed on the server side.

Apparently this is the result of a recent Android update.

Fix: Go to Settings then Data Usage. Hit the menu button and look for “Auto-sync data.” It needs to be checked.

Source: OPSActive.com