Yes, that is correct. If a mixed environment, Unity has to be partnered with the Exchange 2007 server. Honestly, I would schedule some downtime and do a full resync of the directory via the message store and test functionality when failing over to the backup Exchange server. Here is an excerpt of how that should occur:
OK – so the next step is, IMO or from what I’ve been able to find, not well documented. The first time I had to do it, I had that “what the heck is this going to do” feeling. However, it’s not so bad. Here’s where you manually force Unity to sync using the Message Store Configuration wizard. To do this, you must be logged in as a user with rights to the mailstore – personally, I use the UnityInstall account for this task.
7. Open a command line prompt and navigate to the Commserver folder on your server. In my example, I’ve loaded Unity on the D: drive so we’ll be working from that root. So, type D: and enter.
8. Type cd commserverconfigurationsetup and enter.
9. Now type setup.exe sync.
10. The Cisco Unity Message Store Configuration Wizard launches full-screen. Click Next.
11. A process runs and provides a status icon. In short, it says to please wait while Cisco Unity fully resynchronizes with the mailstore. Do not click Next. This process could take quite a long time depending on the size of the database and other factors. For reference, I did this for a customer with about 4000 subscribers and it took approximately 5 hours to complete.
12. When the sync is complete, the Wizard proceeds without prompting and indicates that it is restarting services. When done, click Finish.
If you’re interested to know what goes on in the background, you can take a look at the application log. You’ll notice that the primary directory replication and SQL change writer services stop and start. Once complete, some folks prefer to reboot the Unity server – others do not. Personally, I don't (or have not yet). However, I have seen that it can take a while for things to completely clear up. My recommendation is to create a test subscriber, wait 15-20 minutes for AD, and then verify if the SMTP address is stamped in AD and then in SQL. If it’s not written to SQL right away, be patient and check again within a few hours…you may find it just took a while. Of course, if you still have issues after performing these procedures…it may be time for a TAC case.
The original blog post is related to another issue, but is here for you to browse if you wish. http://www.netcraftsmen.net/resources/blogs/resynchronization-of-unity-w...
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