Best practice is to have the exchange server and unity located in the same site due to performance issues.

PER SRND GUIDE FOR UNITY

To ensure that Cisco Unity functions properly, we require that a Cisco Unity server be in the same data center as the following servers:

•The Exchange server that Cisco Unity communicates with, commonly known as the partner Exchange server. For more information on the partner Exchange server, see the "Exchange Considerations (All Versions)" section.

•Every Exchange server on which mailboxes for that Cisco Unity server are homed. If Cisco Unity is separated by a WAN from an Exchange server on which subscriber mailboxes are homed, a second Cisco Unity server must be co-located with the remote Exchange server.

•At least one domain controller. If Cisco Unity subscribers are homed in more than one domain, a DC for each domain must be in the same data center as the Cisco Unity server.

•At least one global catalog server.

•At least one DNS server.

These requirements apply regardless of whether the non-Cisco Unity servers are customer provided, or are Cisco provided and dedicated to Cisco Unity.

Connect the Cisco Unity server and the servers that Cisco Unity relies on with a high-speed switched gigabit network with no congestion. Total MAPI response time must be less that 10 milliseconds.

The Cisco Unity server should be installed into the same Windows site as the Exchange servers it services.

Yes of course. The unity servers are currently running 4.2 but will be upgraded to 7. Exchange is version 2007. Hope this helps. Thanks.

The requirements are still the same no matter the version of Unity or Partner server. I would recommend staying in line of the recommendation posted above.

To answer your question about what could happen: Phone users accessing messages through the TUI could experience uncomfortable delays while Unity waits for MAPI to return needed data from Exchange (SRND says total MAPI response time must be < 10ms).

Additionally, if Unity decides that Exchange is being to slow to respond, it may assume it has failed and go into fallback mode (there is a term for this; I can't remember it at the moment). This can result in delayed delivery until Exchange responds fast enough for Unity to move the message from local cache to the mailbox.

FWIW, I believe you will not be supported by TAC if the Exchange server does not meet the <10ms transaction response time. Probably not worth the risk in production.

Thank you all for your replies!