Hi,
If you're asking if Unity can partner with anything other than Microsoft
Exchange, or Lotus Domino, the answer is no. However, you can configure
SMTP notifications on Unity that may have an email address on an
external SMTP mail server. As long as the partner Exchange/Domino
server knows how to route these SMTP notifications to the destination,
that's as close as you'll come to what you're looking for if I
understand your question correctly.
Here's the doc for Exchange and SMTP notifications:
http://www.cisco.com/en/US/customer/docs/voice_ip_comm/unity/5x/administ
ration/guide/ex/5xcusag220e.html#wp1047853
Hope that helps,
Brad