One common scenario is that user's passwords expire when they are out of town or remote, and they are unable to change their password. Starting with Exchange 2007 Service Pack 3 (and 2010/2013), users have the ability to change their expired password via OWA if you set a registry key to turn this functionality on.
- On your Exchange CAS server, open regedit and navigate
to HLKM\SYSTEM\CurrentControlSet\Services\MSExchange OWA
- Create a DWORD value called ChangeExpiredPasswordEnabled, and set the value data to 1. If it already exists, simply change the value data to 1.
- Close regedit, open the command prompt and type 'iisreset /noforce' to restart IIS.