No, this is not supported at all and I highly doubt it will work for you.
You can use Unity's web based inbox and use email notification to send user's notifications of new voice messages (which contain links to the web based inbox) which can serve as an integration of sorts for any email server, however doing unified connunications with a back end mail server requires a lot of very detailed interface work be done and tested - the chances of the "Linux open exchange server" emulating all these interfaces for Exchange 5.5/2000 is very slim.
