I've got to give Zimbra credit for how well they executed Free-Busy connectivity with Exchange, a good summary of which and links pertaining thereunto are here:
Exchange looking at Zimbra Free-Busy was no problem, but Zimbra looking at Exchange was generating an error like:
ERROR [EXCHANGE Free/Busy Sync Queue]  fb - cannot modify resourceHowever, you can get it to work. The Magic PowerShell Command on the Exchange side is:
Add-AvailabilityAddressSpace -ForestName [zimbra domain] -AccessMethod PublicFolder
Credit for figuring this out belongs to Eric at Penn. He also used the phrase "Magic PowerShell Command" which I kind of really groove on.
You might also want to check out Microsoft's Implementing Calendar Interoperability (which shows how to do this without lots of coding and judicious use of Exchange Group Policy settings) and Managing Public Folders with the Exchange Management Shell.
Gorier Detail Added September 30, 2008 (again, thanks to Eric at Penn)
To configure Free-Busy from Exchange to Zimbra:
- Create a Service Account on Exchange. Call it "zimbra" (watch your permissions -- see next section)
- Configure Zimbra to connect to the Public Folder Free/Busy interface via this account. You do this on the Zimbra side.
# Specify the Service Account
mcf zimbraFreebusyExchangeAuthScheme form
# Specify the url to Exchange 2007 CAS
# Set the legacydn in Exchange 2007
mcf zimbraFreebusyExchangeUserOrg "/o=First
Organization/ou=Exchange Administrative Group (
Linux folk and Arthur C. Clarke fans: fydibohf23spdlt explained here.
To configure Free-Busy from Zimbra to Exchange:
1. Create a "Zimbra" OU in Active Directory. Make sure all your Zimbra users are in it. But let's define some rules for keeping everyone straight:
- User "Elvis" on Zimbra will in this Active Directory Group be known as "Elvis_Zimbra"
2. Set the Service Account ("zimbra") to update the Free-Busy folder. You do this in PowerShell on the Exchange side.
add-publicfolderclientpermission -identity "\NON_IPM_SUBTREE\SCHEDULE+ FREE
BUSY\EX:/o=First Organization/ou=Exchange Administrative Group
(FYDIBOHF23SPDLT)" -user zimbra -accessrights owner
3. Make Exchange 2007 aware of the Public Folders in the Zimbra domain. You do this in PowerShell on the Exchange side:
Add-AvailabilityAddressSpace -forestname zimbra.YOURDOMAIN.COM -accessmethod publicfolder
4. Update Zimbra accounts to be aware of the email accounts on the AD side. You do this in Zimbra.
# add link from elvis to elvis_zimbra mail contact in AD
ma email@example.com +zimbraForeignPrincipal ad:elvis_zimbra
# add link to OU
ma zimbraFreeBusyExchangeUserOrg "/o=First Organization/ou=Exchange Administrative Group (FYDIBOHF23SPDLT)"