Most corporate mail systems use Microsoft Exchange but enabling POP or IMAP is a security concern (clear text usernames and passwords).

As of Exchange 2007 Exchange Web Services (EWS) http://msdn.microsoft.com/en-us/library/bb408417(EXCHG.80).aspx allows clients to integrate with Exchange mail services securely.

Readily available libraries such as EWSJ http://www.javaexchangeconnector.com/ are available to allow integration with EWS.